Post on 19-Jan-2017
VOLTAGE SAGS MITIGATION TECHNIQUES ANALYSIS
NORSHAFINASH BINTI SAUDIN
A project report submitted in partial fulfillment of the
requirements for the award of the degree of
Master of Engineering (Electrical ndash Power)
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
JUNE 2007
iii
To my beloved husband
iv
ACKNOWLEDGEMENT
I would like to express my gratitude to Allah SWT for giving me the
opportunity to complete this Masterrsquos Project I am deeply indebted to individuals who
directly or indirectly are responsible for this project
I am most grateful to the most kindheartedness supervisor Dr Ahmad Safawi bin
Mokhtar for his guidance in this project and to panel of seminar presentation PM Dr
Mohd Wazir bin Mustafa and PM Md Shah Majid with their superior guidance
information and ideas for this project become abundance
My admiration falls upon En Saudin bin Mat my father and especially to my
mother Pn Siah binti Taharin for them to bear with me my absence in the family Your
encouragement pray and support are very much appreciated
I would also like to express my sincere thanks to my entire friend for their
support and ideas during the development of the project
And last but not the least to my husband thanks
v
ABSTRACT
For some decades power quality did not cause any problem because it had no
effect on most of the loads connected to the electric distribution system When an
induction motor is subjected to voltage sag the motor still operates but with a lower
output until the sag ends With the increased use of sophisticated electronics high
efficiency variable speed drive and power electronic controller power quality has
become an increasing concern to utilities and customers Voltage sags is the most
common type of power quality disturbance in the distribution system It can be caused
by fault in the electrical network or by the starting of a large induction motor Although
the electric utilities have made a substantial amount of investment to improve the
reliability of the network they cannot control the external factor that causes the fault
such as lightning or accumulation of salt at a transmission tower located near to sea
This project intends to investigate mitigation technique that is suitable for different type
of voltage sags source with different type of loads The simulation will be using
PSCADEMTDC software The mitigation techniques that will be studied are such as
Dynamic Voltage Restorer (DVR) Distribution Static Compensator (DSTATCOM) and
Solid State Transfer Switch (SSTS) All the mitigation techniques will be tested on
different type of faults The analysis will focus on the effectiveness of these techniques
in mitigating the voltage sags The study will also investigate the effects of using the
techniques to phase shift At the end of the project it is expected that a few suggestions
can be made on the suitability of the techniques
vi
ABSTRAK
Beberapa dekad yang lalu kualiti kuasa tidak menjadi permasalahan kerana ia
tidak memberi kesan yang sangat nyata kepada beban yang bersambung dengan sistem
pengagihan Apabila motor aruhan mengalami voltan lendut motor tersebut masih
berfungsi tetapi dengan keluaran yang lebih rendah sehingga kejatuhan voltan tamat
Walau bagaimanapun dengan peningkatan penggunaan peralatan elektronik yang maju
pemacu pelbagai halaju berkecekapan tinggi dan pengawal elektronik kuasa kualiti
kuasa mula menjadi perhatian kepada utiliti dan pelanggan Di mana voltan lendut
adalah gangguan kualiti kuasa yang seringkali terjadi terhadap sistem pengagihan yang
disebabkan oleh kerosakan pada rangkaian elektrik dan pemulaan yang besar untuk
motor aruhan Walaupun utiliti telah membuat pelaburan untuk memperbaiki
keboleharapan rangkaian faktor luaran yang menyebabkan kerosakan masih tidak dapat
dikawal contohnya kilat dan pengumpulan garam pada menara penghantaraan yang
terletak berhampiran dengan laut Oleh itu projek ini bertujuan mengkaji kesesuaian
teknik mitigasi untuk pelbagai punca voltan lendut pada beban yang berbeza di mana
perisian PSCADEMTDC digunakan sebagai bantuan untuk simulasi Teknik - teknik
mitigasi yang dikaji adalah seperti Dynamic Voltage Restorer (DVR) Distribution Static
Compensator (DSTATCOM) dan Solid State Transfer Switch (SSTS) Teknik - teknik ini
akan diuji dengan pelbagai kerosakan yang menyebabkan voltan lendut Tumpuan akan
diberikan kepada keberkesanan teknik-teknik tersebut untuk mengatasi voltan lendut dan
kesannya terhadap anjakan fasa Di akhir projek ini beberapa cadangan akan diutarakan
berkenaan kesesuaian teknik - teknik tersebut digunakan untuk mengatasai voltan lendut
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF ABBREVIATIONS xv
LIST OF APPENDICES xvi
I INTRODUCTION 1
11 Introduction 1
12 Problem Statement 3
13 Project Objectives 6
14 Project Scope 6
viii
II VOLTAGE SAGS 7
21 Introduction 7
22 Definition of Voltage Sags 8
23 Standards Associated with Voltage Sags 9
231 IEEE Standard 10
232 Industry Standard 12
2321 SEMI 12
2322 CBEMA (ITI) Curve 14
24 General Causes and Effects of Voltage Sags 15
241 Voltage Sags due to Faults 15
242 Voltage Sags due to Motor Starting 17
243 Voltage Sags due to Transformer Energizing 18
III PSCADEMTDC SOFTWARE 19
31 Introduction 19
32 Characteristics of Software 20
33 Example of Circuit 22
34 Conclusion 25
ix
IV VOLTAGE SAG MITIGATION TECHNIQUES 26
41 Introduction 26
42 Dynamic Voltage Restorer (DVR) 28
421 Principles of DVR Operation 28
43 Distribution Static Compensator (DSTATCOM) 30
421 Basic Configuration and Function of
DSTATCOM 31
44 Solid State Transfer Switch (SSTS) 34
441 Basic Configuration and Function of SSTS 35
V MITIGATION TECNIQUES REALIZATION 39
51 Sinusoidal PWM-Based Control Scheme 39
52 Test System 42
53 Dynamic Voltage Restorer 43
54 Distribution Static Compensator 45
55 Solid State Transfer Switch 47
x
VI SIMULATIONS AND RESULTS 49
61 Test case 49
62 Single line to ground fault 50
621 Phase A to ground 50
622 Phase B to ground 56
623 Phase C to ground 59
63 Double lines to ground fault 62
631 Phase A and B to ground 62
632 Phase A and C to ground 67
633 Phase B and C to ground 70
64 Conclusion 73
VII CONCLUSION 74
71 Conclusion 74
72 Suggestion 77
REFERENCES 78
Appendices A-C 81-85
xi
LIST OF TABLES
TABLE NO TITLE PAGE
11 Cause of TNB network disruption 4
61 (a) Test results for line A to the ground fault (b) Recovery result 5
62 (a) Test results for line B to the ground fault (b) Recovery result 8
63 (a) Test results for line C to the ground fault (b) Recovery result 1
64 (a) Test results for line AB to the ground fault (b) Recovery result 6
65 (a) Test results for line AC to the ground fault (b) Recovery result 9
66 (a) Test results for line BC to the ground fault (b) Recovery result 2
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
iii
To my beloved husband
iv
ACKNOWLEDGEMENT
I would like to express my gratitude to Allah SWT for giving me the
opportunity to complete this Masterrsquos Project I am deeply indebted to individuals who
directly or indirectly are responsible for this project
I am most grateful to the most kindheartedness supervisor Dr Ahmad Safawi bin
Mokhtar for his guidance in this project and to panel of seminar presentation PM Dr
Mohd Wazir bin Mustafa and PM Md Shah Majid with their superior guidance
information and ideas for this project become abundance
My admiration falls upon En Saudin bin Mat my father and especially to my
mother Pn Siah binti Taharin for them to bear with me my absence in the family Your
encouragement pray and support are very much appreciated
I would also like to express my sincere thanks to my entire friend for their
support and ideas during the development of the project
And last but not the least to my husband thanks
v
ABSTRACT
For some decades power quality did not cause any problem because it had no
effect on most of the loads connected to the electric distribution system When an
induction motor is subjected to voltage sag the motor still operates but with a lower
output until the sag ends With the increased use of sophisticated electronics high
efficiency variable speed drive and power electronic controller power quality has
become an increasing concern to utilities and customers Voltage sags is the most
common type of power quality disturbance in the distribution system It can be caused
by fault in the electrical network or by the starting of a large induction motor Although
the electric utilities have made a substantial amount of investment to improve the
reliability of the network they cannot control the external factor that causes the fault
such as lightning or accumulation of salt at a transmission tower located near to sea
This project intends to investigate mitigation technique that is suitable for different type
of voltage sags source with different type of loads The simulation will be using
PSCADEMTDC software The mitigation techniques that will be studied are such as
Dynamic Voltage Restorer (DVR) Distribution Static Compensator (DSTATCOM) and
Solid State Transfer Switch (SSTS) All the mitigation techniques will be tested on
different type of faults The analysis will focus on the effectiveness of these techniques
in mitigating the voltage sags The study will also investigate the effects of using the
techniques to phase shift At the end of the project it is expected that a few suggestions
can be made on the suitability of the techniques
vi
ABSTRAK
Beberapa dekad yang lalu kualiti kuasa tidak menjadi permasalahan kerana ia
tidak memberi kesan yang sangat nyata kepada beban yang bersambung dengan sistem
pengagihan Apabila motor aruhan mengalami voltan lendut motor tersebut masih
berfungsi tetapi dengan keluaran yang lebih rendah sehingga kejatuhan voltan tamat
Walau bagaimanapun dengan peningkatan penggunaan peralatan elektronik yang maju
pemacu pelbagai halaju berkecekapan tinggi dan pengawal elektronik kuasa kualiti
kuasa mula menjadi perhatian kepada utiliti dan pelanggan Di mana voltan lendut
adalah gangguan kualiti kuasa yang seringkali terjadi terhadap sistem pengagihan yang
disebabkan oleh kerosakan pada rangkaian elektrik dan pemulaan yang besar untuk
motor aruhan Walaupun utiliti telah membuat pelaburan untuk memperbaiki
keboleharapan rangkaian faktor luaran yang menyebabkan kerosakan masih tidak dapat
dikawal contohnya kilat dan pengumpulan garam pada menara penghantaraan yang
terletak berhampiran dengan laut Oleh itu projek ini bertujuan mengkaji kesesuaian
teknik mitigasi untuk pelbagai punca voltan lendut pada beban yang berbeza di mana
perisian PSCADEMTDC digunakan sebagai bantuan untuk simulasi Teknik - teknik
mitigasi yang dikaji adalah seperti Dynamic Voltage Restorer (DVR) Distribution Static
Compensator (DSTATCOM) dan Solid State Transfer Switch (SSTS) Teknik - teknik ini
akan diuji dengan pelbagai kerosakan yang menyebabkan voltan lendut Tumpuan akan
diberikan kepada keberkesanan teknik-teknik tersebut untuk mengatasi voltan lendut dan
kesannya terhadap anjakan fasa Di akhir projek ini beberapa cadangan akan diutarakan
berkenaan kesesuaian teknik - teknik tersebut digunakan untuk mengatasai voltan lendut
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF ABBREVIATIONS xv
LIST OF APPENDICES xvi
I INTRODUCTION 1
11 Introduction 1
12 Problem Statement 3
13 Project Objectives 6
14 Project Scope 6
viii
II VOLTAGE SAGS 7
21 Introduction 7
22 Definition of Voltage Sags 8
23 Standards Associated with Voltage Sags 9
231 IEEE Standard 10
232 Industry Standard 12
2321 SEMI 12
2322 CBEMA (ITI) Curve 14
24 General Causes and Effects of Voltage Sags 15
241 Voltage Sags due to Faults 15
242 Voltage Sags due to Motor Starting 17
243 Voltage Sags due to Transformer Energizing 18
III PSCADEMTDC SOFTWARE 19
31 Introduction 19
32 Characteristics of Software 20
33 Example of Circuit 22
34 Conclusion 25
ix
IV VOLTAGE SAG MITIGATION TECHNIQUES 26
41 Introduction 26
42 Dynamic Voltage Restorer (DVR) 28
421 Principles of DVR Operation 28
43 Distribution Static Compensator (DSTATCOM) 30
421 Basic Configuration and Function of
DSTATCOM 31
44 Solid State Transfer Switch (SSTS) 34
441 Basic Configuration and Function of SSTS 35
V MITIGATION TECNIQUES REALIZATION 39
51 Sinusoidal PWM-Based Control Scheme 39
52 Test System 42
53 Dynamic Voltage Restorer 43
54 Distribution Static Compensator 45
55 Solid State Transfer Switch 47
x
VI SIMULATIONS AND RESULTS 49
61 Test case 49
62 Single line to ground fault 50
621 Phase A to ground 50
622 Phase B to ground 56
623 Phase C to ground 59
63 Double lines to ground fault 62
631 Phase A and B to ground 62
632 Phase A and C to ground 67
633 Phase B and C to ground 70
64 Conclusion 73
VII CONCLUSION 74
71 Conclusion 74
72 Suggestion 77
REFERENCES 78
Appendices A-C 81-85
xi
LIST OF TABLES
TABLE NO TITLE PAGE
11 Cause of TNB network disruption 4
61 (a) Test results for line A to the ground fault (b) Recovery result 5
62 (a) Test results for line B to the ground fault (b) Recovery result 8
63 (a) Test results for line C to the ground fault (b) Recovery result 1
64 (a) Test results for line AB to the ground fault (b) Recovery result 6
65 (a) Test results for line AC to the ground fault (b) Recovery result 9
66 (a) Test results for line BC to the ground fault (b) Recovery result 2
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
iv
ACKNOWLEDGEMENT
I would like to express my gratitude to Allah SWT for giving me the
opportunity to complete this Masterrsquos Project I am deeply indebted to individuals who
directly or indirectly are responsible for this project
I am most grateful to the most kindheartedness supervisor Dr Ahmad Safawi bin
Mokhtar for his guidance in this project and to panel of seminar presentation PM Dr
Mohd Wazir bin Mustafa and PM Md Shah Majid with their superior guidance
information and ideas for this project become abundance
My admiration falls upon En Saudin bin Mat my father and especially to my
mother Pn Siah binti Taharin for them to bear with me my absence in the family Your
encouragement pray and support are very much appreciated
I would also like to express my sincere thanks to my entire friend for their
support and ideas during the development of the project
And last but not the least to my husband thanks
v
ABSTRACT
For some decades power quality did not cause any problem because it had no
effect on most of the loads connected to the electric distribution system When an
induction motor is subjected to voltage sag the motor still operates but with a lower
output until the sag ends With the increased use of sophisticated electronics high
efficiency variable speed drive and power electronic controller power quality has
become an increasing concern to utilities and customers Voltage sags is the most
common type of power quality disturbance in the distribution system It can be caused
by fault in the electrical network or by the starting of a large induction motor Although
the electric utilities have made a substantial amount of investment to improve the
reliability of the network they cannot control the external factor that causes the fault
such as lightning or accumulation of salt at a transmission tower located near to sea
This project intends to investigate mitigation technique that is suitable for different type
of voltage sags source with different type of loads The simulation will be using
PSCADEMTDC software The mitigation techniques that will be studied are such as
Dynamic Voltage Restorer (DVR) Distribution Static Compensator (DSTATCOM) and
Solid State Transfer Switch (SSTS) All the mitigation techniques will be tested on
different type of faults The analysis will focus on the effectiveness of these techniques
in mitigating the voltage sags The study will also investigate the effects of using the
techniques to phase shift At the end of the project it is expected that a few suggestions
can be made on the suitability of the techniques
vi
ABSTRAK
Beberapa dekad yang lalu kualiti kuasa tidak menjadi permasalahan kerana ia
tidak memberi kesan yang sangat nyata kepada beban yang bersambung dengan sistem
pengagihan Apabila motor aruhan mengalami voltan lendut motor tersebut masih
berfungsi tetapi dengan keluaran yang lebih rendah sehingga kejatuhan voltan tamat
Walau bagaimanapun dengan peningkatan penggunaan peralatan elektronik yang maju
pemacu pelbagai halaju berkecekapan tinggi dan pengawal elektronik kuasa kualiti
kuasa mula menjadi perhatian kepada utiliti dan pelanggan Di mana voltan lendut
adalah gangguan kualiti kuasa yang seringkali terjadi terhadap sistem pengagihan yang
disebabkan oleh kerosakan pada rangkaian elektrik dan pemulaan yang besar untuk
motor aruhan Walaupun utiliti telah membuat pelaburan untuk memperbaiki
keboleharapan rangkaian faktor luaran yang menyebabkan kerosakan masih tidak dapat
dikawal contohnya kilat dan pengumpulan garam pada menara penghantaraan yang
terletak berhampiran dengan laut Oleh itu projek ini bertujuan mengkaji kesesuaian
teknik mitigasi untuk pelbagai punca voltan lendut pada beban yang berbeza di mana
perisian PSCADEMTDC digunakan sebagai bantuan untuk simulasi Teknik - teknik
mitigasi yang dikaji adalah seperti Dynamic Voltage Restorer (DVR) Distribution Static
Compensator (DSTATCOM) dan Solid State Transfer Switch (SSTS) Teknik - teknik ini
akan diuji dengan pelbagai kerosakan yang menyebabkan voltan lendut Tumpuan akan
diberikan kepada keberkesanan teknik-teknik tersebut untuk mengatasi voltan lendut dan
kesannya terhadap anjakan fasa Di akhir projek ini beberapa cadangan akan diutarakan
berkenaan kesesuaian teknik - teknik tersebut digunakan untuk mengatasai voltan lendut
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF ABBREVIATIONS xv
LIST OF APPENDICES xvi
I INTRODUCTION 1
11 Introduction 1
12 Problem Statement 3
13 Project Objectives 6
14 Project Scope 6
viii
II VOLTAGE SAGS 7
21 Introduction 7
22 Definition of Voltage Sags 8
23 Standards Associated with Voltage Sags 9
231 IEEE Standard 10
232 Industry Standard 12
2321 SEMI 12
2322 CBEMA (ITI) Curve 14
24 General Causes and Effects of Voltage Sags 15
241 Voltage Sags due to Faults 15
242 Voltage Sags due to Motor Starting 17
243 Voltage Sags due to Transformer Energizing 18
III PSCADEMTDC SOFTWARE 19
31 Introduction 19
32 Characteristics of Software 20
33 Example of Circuit 22
34 Conclusion 25
ix
IV VOLTAGE SAG MITIGATION TECHNIQUES 26
41 Introduction 26
42 Dynamic Voltage Restorer (DVR) 28
421 Principles of DVR Operation 28
43 Distribution Static Compensator (DSTATCOM) 30
421 Basic Configuration and Function of
DSTATCOM 31
44 Solid State Transfer Switch (SSTS) 34
441 Basic Configuration and Function of SSTS 35
V MITIGATION TECNIQUES REALIZATION 39
51 Sinusoidal PWM-Based Control Scheme 39
52 Test System 42
53 Dynamic Voltage Restorer 43
54 Distribution Static Compensator 45
55 Solid State Transfer Switch 47
x
VI SIMULATIONS AND RESULTS 49
61 Test case 49
62 Single line to ground fault 50
621 Phase A to ground 50
622 Phase B to ground 56
623 Phase C to ground 59
63 Double lines to ground fault 62
631 Phase A and B to ground 62
632 Phase A and C to ground 67
633 Phase B and C to ground 70
64 Conclusion 73
VII CONCLUSION 74
71 Conclusion 74
72 Suggestion 77
REFERENCES 78
Appendices A-C 81-85
xi
LIST OF TABLES
TABLE NO TITLE PAGE
11 Cause of TNB network disruption 4
61 (a) Test results for line A to the ground fault (b) Recovery result 5
62 (a) Test results for line B to the ground fault (b) Recovery result 8
63 (a) Test results for line C to the ground fault (b) Recovery result 1
64 (a) Test results for line AB to the ground fault (b) Recovery result 6
65 (a) Test results for line AC to the ground fault (b) Recovery result 9
66 (a) Test results for line BC to the ground fault (b) Recovery result 2
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
v
ABSTRACT
For some decades power quality did not cause any problem because it had no
effect on most of the loads connected to the electric distribution system When an
induction motor is subjected to voltage sag the motor still operates but with a lower
output until the sag ends With the increased use of sophisticated electronics high
efficiency variable speed drive and power electronic controller power quality has
become an increasing concern to utilities and customers Voltage sags is the most
common type of power quality disturbance in the distribution system It can be caused
by fault in the electrical network or by the starting of a large induction motor Although
the electric utilities have made a substantial amount of investment to improve the
reliability of the network they cannot control the external factor that causes the fault
such as lightning or accumulation of salt at a transmission tower located near to sea
This project intends to investigate mitigation technique that is suitable for different type
of voltage sags source with different type of loads The simulation will be using
PSCADEMTDC software The mitigation techniques that will be studied are such as
Dynamic Voltage Restorer (DVR) Distribution Static Compensator (DSTATCOM) and
Solid State Transfer Switch (SSTS) All the mitigation techniques will be tested on
different type of faults The analysis will focus on the effectiveness of these techniques
in mitigating the voltage sags The study will also investigate the effects of using the
techniques to phase shift At the end of the project it is expected that a few suggestions
can be made on the suitability of the techniques
vi
ABSTRAK
Beberapa dekad yang lalu kualiti kuasa tidak menjadi permasalahan kerana ia
tidak memberi kesan yang sangat nyata kepada beban yang bersambung dengan sistem
pengagihan Apabila motor aruhan mengalami voltan lendut motor tersebut masih
berfungsi tetapi dengan keluaran yang lebih rendah sehingga kejatuhan voltan tamat
Walau bagaimanapun dengan peningkatan penggunaan peralatan elektronik yang maju
pemacu pelbagai halaju berkecekapan tinggi dan pengawal elektronik kuasa kualiti
kuasa mula menjadi perhatian kepada utiliti dan pelanggan Di mana voltan lendut
adalah gangguan kualiti kuasa yang seringkali terjadi terhadap sistem pengagihan yang
disebabkan oleh kerosakan pada rangkaian elektrik dan pemulaan yang besar untuk
motor aruhan Walaupun utiliti telah membuat pelaburan untuk memperbaiki
keboleharapan rangkaian faktor luaran yang menyebabkan kerosakan masih tidak dapat
dikawal contohnya kilat dan pengumpulan garam pada menara penghantaraan yang
terletak berhampiran dengan laut Oleh itu projek ini bertujuan mengkaji kesesuaian
teknik mitigasi untuk pelbagai punca voltan lendut pada beban yang berbeza di mana
perisian PSCADEMTDC digunakan sebagai bantuan untuk simulasi Teknik - teknik
mitigasi yang dikaji adalah seperti Dynamic Voltage Restorer (DVR) Distribution Static
Compensator (DSTATCOM) dan Solid State Transfer Switch (SSTS) Teknik - teknik ini
akan diuji dengan pelbagai kerosakan yang menyebabkan voltan lendut Tumpuan akan
diberikan kepada keberkesanan teknik-teknik tersebut untuk mengatasi voltan lendut dan
kesannya terhadap anjakan fasa Di akhir projek ini beberapa cadangan akan diutarakan
berkenaan kesesuaian teknik - teknik tersebut digunakan untuk mengatasai voltan lendut
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF ABBREVIATIONS xv
LIST OF APPENDICES xvi
I INTRODUCTION 1
11 Introduction 1
12 Problem Statement 3
13 Project Objectives 6
14 Project Scope 6
viii
II VOLTAGE SAGS 7
21 Introduction 7
22 Definition of Voltage Sags 8
23 Standards Associated with Voltage Sags 9
231 IEEE Standard 10
232 Industry Standard 12
2321 SEMI 12
2322 CBEMA (ITI) Curve 14
24 General Causes and Effects of Voltage Sags 15
241 Voltage Sags due to Faults 15
242 Voltage Sags due to Motor Starting 17
243 Voltage Sags due to Transformer Energizing 18
III PSCADEMTDC SOFTWARE 19
31 Introduction 19
32 Characteristics of Software 20
33 Example of Circuit 22
34 Conclusion 25
ix
IV VOLTAGE SAG MITIGATION TECHNIQUES 26
41 Introduction 26
42 Dynamic Voltage Restorer (DVR) 28
421 Principles of DVR Operation 28
43 Distribution Static Compensator (DSTATCOM) 30
421 Basic Configuration and Function of
DSTATCOM 31
44 Solid State Transfer Switch (SSTS) 34
441 Basic Configuration and Function of SSTS 35
V MITIGATION TECNIQUES REALIZATION 39
51 Sinusoidal PWM-Based Control Scheme 39
52 Test System 42
53 Dynamic Voltage Restorer 43
54 Distribution Static Compensator 45
55 Solid State Transfer Switch 47
x
VI SIMULATIONS AND RESULTS 49
61 Test case 49
62 Single line to ground fault 50
621 Phase A to ground 50
622 Phase B to ground 56
623 Phase C to ground 59
63 Double lines to ground fault 62
631 Phase A and B to ground 62
632 Phase A and C to ground 67
633 Phase B and C to ground 70
64 Conclusion 73
VII CONCLUSION 74
71 Conclusion 74
72 Suggestion 77
REFERENCES 78
Appendices A-C 81-85
xi
LIST OF TABLES
TABLE NO TITLE PAGE
11 Cause of TNB network disruption 4
61 (a) Test results for line A to the ground fault (b) Recovery result 5
62 (a) Test results for line B to the ground fault (b) Recovery result 8
63 (a) Test results for line C to the ground fault (b) Recovery result 1
64 (a) Test results for line AB to the ground fault (b) Recovery result 6
65 (a) Test results for line AC to the ground fault (b) Recovery result 9
66 (a) Test results for line BC to the ground fault (b) Recovery result 2
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
vi
ABSTRAK
Beberapa dekad yang lalu kualiti kuasa tidak menjadi permasalahan kerana ia
tidak memberi kesan yang sangat nyata kepada beban yang bersambung dengan sistem
pengagihan Apabila motor aruhan mengalami voltan lendut motor tersebut masih
berfungsi tetapi dengan keluaran yang lebih rendah sehingga kejatuhan voltan tamat
Walau bagaimanapun dengan peningkatan penggunaan peralatan elektronik yang maju
pemacu pelbagai halaju berkecekapan tinggi dan pengawal elektronik kuasa kualiti
kuasa mula menjadi perhatian kepada utiliti dan pelanggan Di mana voltan lendut
adalah gangguan kualiti kuasa yang seringkali terjadi terhadap sistem pengagihan yang
disebabkan oleh kerosakan pada rangkaian elektrik dan pemulaan yang besar untuk
motor aruhan Walaupun utiliti telah membuat pelaburan untuk memperbaiki
keboleharapan rangkaian faktor luaran yang menyebabkan kerosakan masih tidak dapat
dikawal contohnya kilat dan pengumpulan garam pada menara penghantaraan yang
terletak berhampiran dengan laut Oleh itu projek ini bertujuan mengkaji kesesuaian
teknik mitigasi untuk pelbagai punca voltan lendut pada beban yang berbeza di mana
perisian PSCADEMTDC digunakan sebagai bantuan untuk simulasi Teknik - teknik
mitigasi yang dikaji adalah seperti Dynamic Voltage Restorer (DVR) Distribution Static
Compensator (DSTATCOM) dan Solid State Transfer Switch (SSTS) Teknik - teknik ini
akan diuji dengan pelbagai kerosakan yang menyebabkan voltan lendut Tumpuan akan
diberikan kepada keberkesanan teknik-teknik tersebut untuk mengatasi voltan lendut dan
kesannya terhadap anjakan fasa Di akhir projek ini beberapa cadangan akan diutarakan
berkenaan kesesuaian teknik - teknik tersebut digunakan untuk mengatasai voltan lendut
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF ABBREVIATIONS xv
LIST OF APPENDICES xvi
I INTRODUCTION 1
11 Introduction 1
12 Problem Statement 3
13 Project Objectives 6
14 Project Scope 6
viii
II VOLTAGE SAGS 7
21 Introduction 7
22 Definition of Voltage Sags 8
23 Standards Associated with Voltage Sags 9
231 IEEE Standard 10
232 Industry Standard 12
2321 SEMI 12
2322 CBEMA (ITI) Curve 14
24 General Causes and Effects of Voltage Sags 15
241 Voltage Sags due to Faults 15
242 Voltage Sags due to Motor Starting 17
243 Voltage Sags due to Transformer Energizing 18
III PSCADEMTDC SOFTWARE 19
31 Introduction 19
32 Characteristics of Software 20
33 Example of Circuit 22
34 Conclusion 25
ix
IV VOLTAGE SAG MITIGATION TECHNIQUES 26
41 Introduction 26
42 Dynamic Voltage Restorer (DVR) 28
421 Principles of DVR Operation 28
43 Distribution Static Compensator (DSTATCOM) 30
421 Basic Configuration and Function of
DSTATCOM 31
44 Solid State Transfer Switch (SSTS) 34
441 Basic Configuration and Function of SSTS 35
V MITIGATION TECNIQUES REALIZATION 39
51 Sinusoidal PWM-Based Control Scheme 39
52 Test System 42
53 Dynamic Voltage Restorer 43
54 Distribution Static Compensator 45
55 Solid State Transfer Switch 47
x
VI SIMULATIONS AND RESULTS 49
61 Test case 49
62 Single line to ground fault 50
621 Phase A to ground 50
622 Phase B to ground 56
623 Phase C to ground 59
63 Double lines to ground fault 62
631 Phase A and B to ground 62
632 Phase A and C to ground 67
633 Phase B and C to ground 70
64 Conclusion 73
VII CONCLUSION 74
71 Conclusion 74
72 Suggestion 77
REFERENCES 78
Appendices A-C 81-85
xi
LIST OF TABLES
TABLE NO TITLE PAGE
11 Cause of TNB network disruption 4
61 (a) Test results for line A to the ground fault (b) Recovery result 5
62 (a) Test results for line B to the ground fault (b) Recovery result 8
63 (a) Test results for line C to the ground fault (b) Recovery result 1
64 (a) Test results for line AB to the ground fault (b) Recovery result 6
65 (a) Test results for line AC to the ground fault (b) Recovery result 9
66 (a) Test results for line BC to the ground fault (b) Recovery result 2
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF ABBREVIATIONS xv
LIST OF APPENDICES xvi
I INTRODUCTION 1
11 Introduction 1
12 Problem Statement 3
13 Project Objectives 6
14 Project Scope 6
viii
II VOLTAGE SAGS 7
21 Introduction 7
22 Definition of Voltage Sags 8
23 Standards Associated with Voltage Sags 9
231 IEEE Standard 10
232 Industry Standard 12
2321 SEMI 12
2322 CBEMA (ITI) Curve 14
24 General Causes and Effects of Voltage Sags 15
241 Voltage Sags due to Faults 15
242 Voltage Sags due to Motor Starting 17
243 Voltage Sags due to Transformer Energizing 18
III PSCADEMTDC SOFTWARE 19
31 Introduction 19
32 Characteristics of Software 20
33 Example of Circuit 22
34 Conclusion 25
ix
IV VOLTAGE SAG MITIGATION TECHNIQUES 26
41 Introduction 26
42 Dynamic Voltage Restorer (DVR) 28
421 Principles of DVR Operation 28
43 Distribution Static Compensator (DSTATCOM) 30
421 Basic Configuration and Function of
DSTATCOM 31
44 Solid State Transfer Switch (SSTS) 34
441 Basic Configuration and Function of SSTS 35
V MITIGATION TECNIQUES REALIZATION 39
51 Sinusoidal PWM-Based Control Scheme 39
52 Test System 42
53 Dynamic Voltage Restorer 43
54 Distribution Static Compensator 45
55 Solid State Transfer Switch 47
x
VI SIMULATIONS AND RESULTS 49
61 Test case 49
62 Single line to ground fault 50
621 Phase A to ground 50
622 Phase B to ground 56
623 Phase C to ground 59
63 Double lines to ground fault 62
631 Phase A and B to ground 62
632 Phase A and C to ground 67
633 Phase B and C to ground 70
64 Conclusion 73
VII CONCLUSION 74
71 Conclusion 74
72 Suggestion 77
REFERENCES 78
Appendices A-C 81-85
xi
LIST OF TABLES
TABLE NO TITLE PAGE
11 Cause of TNB network disruption 4
61 (a) Test results for line A to the ground fault (b) Recovery result 5
62 (a) Test results for line B to the ground fault (b) Recovery result 8
63 (a) Test results for line C to the ground fault (b) Recovery result 1
64 (a) Test results for line AB to the ground fault (b) Recovery result 6
65 (a) Test results for line AC to the ground fault (b) Recovery result 9
66 (a) Test results for line BC to the ground fault (b) Recovery result 2
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
viii
II VOLTAGE SAGS 7
21 Introduction 7
22 Definition of Voltage Sags 8
23 Standards Associated with Voltage Sags 9
231 IEEE Standard 10
232 Industry Standard 12
2321 SEMI 12
2322 CBEMA (ITI) Curve 14
24 General Causes and Effects of Voltage Sags 15
241 Voltage Sags due to Faults 15
242 Voltage Sags due to Motor Starting 17
243 Voltage Sags due to Transformer Energizing 18
III PSCADEMTDC SOFTWARE 19
31 Introduction 19
32 Characteristics of Software 20
33 Example of Circuit 22
34 Conclusion 25
ix
IV VOLTAGE SAG MITIGATION TECHNIQUES 26
41 Introduction 26
42 Dynamic Voltage Restorer (DVR) 28
421 Principles of DVR Operation 28
43 Distribution Static Compensator (DSTATCOM) 30
421 Basic Configuration and Function of
DSTATCOM 31
44 Solid State Transfer Switch (SSTS) 34
441 Basic Configuration and Function of SSTS 35
V MITIGATION TECNIQUES REALIZATION 39
51 Sinusoidal PWM-Based Control Scheme 39
52 Test System 42
53 Dynamic Voltage Restorer 43
54 Distribution Static Compensator 45
55 Solid State Transfer Switch 47
x
VI SIMULATIONS AND RESULTS 49
61 Test case 49
62 Single line to ground fault 50
621 Phase A to ground 50
622 Phase B to ground 56
623 Phase C to ground 59
63 Double lines to ground fault 62
631 Phase A and B to ground 62
632 Phase A and C to ground 67
633 Phase B and C to ground 70
64 Conclusion 73
VII CONCLUSION 74
71 Conclusion 74
72 Suggestion 77
REFERENCES 78
Appendices A-C 81-85
xi
LIST OF TABLES
TABLE NO TITLE PAGE
11 Cause of TNB network disruption 4
61 (a) Test results for line A to the ground fault (b) Recovery result 5
62 (a) Test results for line B to the ground fault (b) Recovery result 8
63 (a) Test results for line C to the ground fault (b) Recovery result 1
64 (a) Test results for line AB to the ground fault (b) Recovery result 6
65 (a) Test results for line AC to the ground fault (b) Recovery result 9
66 (a) Test results for line BC to the ground fault (b) Recovery result 2
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
ix
IV VOLTAGE SAG MITIGATION TECHNIQUES 26
41 Introduction 26
42 Dynamic Voltage Restorer (DVR) 28
421 Principles of DVR Operation 28
43 Distribution Static Compensator (DSTATCOM) 30
421 Basic Configuration and Function of
DSTATCOM 31
44 Solid State Transfer Switch (SSTS) 34
441 Basic Configuration and Function of SSTS 35
V MITIGATION TECNIQUES REALIZATION 39
51 Sinusoidal PWM-Based Control Scheme 39
52 Test System 42
53 Dynamic Voltage Restorer 43
54 Distribution Static Compensator 45
55 Solid State Transfer Switch 47
x
VI SIMULATIONS AND RESULTS 49
61 Test case 49
62 Single line to ground fault 50
621 Phase A to ground 50
622 Phase B to ground 56
623 Phase C to ground 59
63 Double lines to ground fault 62
631 Phase A and B to ground 62
632 Phase A and C to ground 67
633 Phase B and C to ground 70
64 Conclusion 73
VII CONCLUSION 74
71 Conclusion 74
72 Suggestion 77
REFERENCES 78
Appendices A-C 81-85
xi
LIST OF TABLES
TABLE NO TITLE PAGE
11 Cause of TNB network disruption 4
61 (a) Test results for line A to the ground fault (b) Recovery result 5
62 (a) Test results for line B to the ground fault (b) Recovery result 8
63 (a) Test results for line C to the ground fault (b) Recovery result 1
64 (a) Test results for line AB to the ground fault (b) Recovery result 6
65 (a) Test results for line AC to the ground fault (b) Recovery result 9
66 (a) Test results for line BC to the ground fault (b) Recovery result 2
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
x
VI SIMULATIONS AND RESULTS 49
61 Test case 49
62 Single line to ground fault 50
621 Phase A to ground 50
622 Phase B to ground 56
623 Phase C to ground 59
63 Double lines to ground fault 62
631 Phase A and B to ground 62
632 Phase A and C to ground 67
633 Phase B and C to ground 70
64 Conclusion 73
VII CONCLUSION 74
71 Conclusion 74
72 Suggestion 77
REFERENCES 78
Appendices A-C 81-85
xi
LIST OF TABLES
TABLE NO TITLE PAGE
11 Cause of TNB network disruption 4
61 (a) Test results for line A to the ground fault (b) Recovery result 5
62 (a) Test results for line B to the ground fault (b) Recovery result 8
63 (a) Test results for line C to the ground fault (b) Recovery result 1
64 (a) Test results for line AB to the ground fault (b) Recovery result 6
65 (a) Test results for line AC to the ground fault (b) Recovery result 9
66 (a) Test results for line BC to the ground fault (b) Recovery result 2
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
xi
LIST OF TABLES
TABLE NO TITLE PAGE
11 Cause of TNB network disruption 4
61 (a) Test results for line A to the ground fault (b) Recovery result 5
62 (a) Test results for line B to the ground fault (b) Recovery result 8
63 (a) Test results for line C to the ground fault (b) Recovery result 1
64 (a) Test results for line AB to the ground fault (b) Recovery result 6
65 (a) Test results for line AC to the ground fault (b) Recovery result 9
66 (a) Test results for line BC to the ground fault (b) Recovery result 2
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
xii
LIST OF FIGURES
FIGURE NO TITLE PAGE
11 Demarcation of the various power quality issues defined
by IEEE Std 1159-1995 2
21 Depiction of voltage sag 9
22 Immunity curve for semiconductor manufacturing
equipment according to SEMI F47 13
23 Revised CBEMA curve ITIC curve 1996 14
24 Voltage sag due to a cleared line-ground fault 16
25 Voltage sag due to motor starting 17
26 Voltage sag due to transformer energizing 18
31 DVR with main components in PSCAD 23
32 The Wye-Connected DVR in PSCAD 24
41 Different protection options for improving performance during
power quality variation 27
42 Principle of DVR with a response time of less than one
millisecond 29
43 Schematic diagram of the DSTATCOM as a custom
power controller 30
44 Building blocks of DSTATCOM 32
45 Operation modes of a DSTATCOM 33
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
xiii
46 Schematic representations of the SSTS as a custom power device 34
47 Solid State Transfer Switch systems 35
48 Thyristors of the SSTS conducting in the positive and
negative half cycle of the preferred source 37
49 Thyristors on the alternate supply are turned ON on sensing
a disturbance on the preferred source 38
51 Control scheme for the test system implemented in
PSCADEMTDC to carry out the DSTATCOM and DVR
simulations 40
52 The test system implemented in PSCADEMTDC 42
53 One line diagram of the DVR test system 43
54 Schematic diagram of the DVR 44
55 Schematic diagram of the test system with DVR connected
to the system 44
56 One line diagram of the DSTATCOM test system 45
57 Schematic diagram of the test system with DSTATCOM
connected to the system 46
58 One line diagram of the SSTS test system 47
59 SSTS switches implemented in PSCADEMTDC 48
510 Schematic diagram of the test system with SSTS connected
to the system 48
61 (a) Phase shift for line A to the ground fault
(b) Rms voltage drop 50
62 (a) Corrected phase with DVR
(b) Compensated voltage sag with DVR 51
63 (a) Corrected phase using DSTATCOM
(b) Compensated voltage sag using DSTATCOM 53
64 (a) Corrected phase using SSTS
(b) Compensated voltage sag using SSTS 54
65 Phase shift of line B to the ground fault 56
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
xiv
66 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B to
the ground fault 57
67 Phase shift of line B to the ground fault 59
68 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line C to
the ground fault 60
69 (a) Phase shift for line A and B to the ground fault
(b) Rms voltage drop 63
610 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and B
to the ground fault 64
611 (a) Compensated voltage sag using DVR
(b) Compensated voltage sag using DSTATCOM
Line A and B to the ground fault 65
612 Phase shift for line A and C to the ground fault 67
613 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line A and C
to the ground fault 68
614 Phase shift for line B and C to the ground fault 70
615 (a) Phase correction using DVR
(b) Phase correction using DSTATCOM line B and C
to the ground fault 71
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
xv
LIST OF ABBREVIATIONS
CBEMA - Computer Business Equipment Manufacturers Association
DSTATCOM - Distribution Static Compensator
DVR - Dynamic Voltage Restorer
EMTDC - Electromagnetic Transient Program with DC Analysis
ERM - Electronic Restart Modules
Hz - Hertz
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronics Engineers
ITIC - Information Technology Industry Council
kV - kilovolt
MVA - megavolt ampere
MVAR - mega volt amps reactive
MW - megawatt
pu - per unit
PCC - point of common coupling
PSCAD - Power System Aided Design
PWM - Pulse Width Modulation
RMS - root mean square
SEMI - Semiconductor Equipment and Materials International
SSTS - Solid State Transfer Switch
TNB - Tenaga Nasional Berhad
TRV - transient recovery voltage
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
xvi
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Data generated by PSCADEMTDC for DSTATCOM 81
B Data generated by PSCADEMTDC for DVR 83
C Data generated by PSCADEMTDC for SSTS 85
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
CHAPTER I
INTRODUCTION
11 Introduction
Both electric utilities and end users of electrical power are becoming increasingly
concerned about the quality of electric power The term power quality has become one
of the most prolific buzzword in the power industry since the late 1980s [1] The issue in
electricity power sector delivery is not confined to only energy efficiency and
environment but more importantly on quality and continuity of supply or power quality
and supply quality Electrical Power quality is the degree of any deviation from the
nominal values of the voltage magnitude and frequency Power quality may also be
defined as the degree to which both the utilization and delivery of electric power affects
the performance of electrical equipment [2] From a customer perspective a power
quality problem is defined as any power problem manifested in voltage current or
frequency deviations that result in power failure or disoperation of customer of
equipment [3]
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
2
Power quality problems concerning frequency deviation are the presence of
harmonics and other departures from the intended frequency of the alternating supply
voltage On the other hand power quality problems concerning voltage magnitude
deviations can be in the form of voltage fluctuations especially those causing flicker
Other voltage problems are the voltage sags short interruptions and transient over
voltages Transient over voltage has some of the characteristics of high-frequency
phenomena In a three-phase system unbalanced voltages also is a power quality
problem [2] Among them two power quality problems have been identified to be of
major concern to the customers are voltage sags and harmonics but this project will be
focusing on voltage sags
Figures 11 describe the demarcation of the various power quality issues defined
by IEEE Std 1159-1995 [4]
Figure 11 Demarcation of the various power quality issues defined by IEEE
Std 1159-1995[4]
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
3
Three factors that are driving interest and serious concerns in power quality are
[1]
i Increased load sensitivity and production automation The focus on
power quality is therefore more of voltage quality as the momentary drop
in voltage disrupts automated manufacturing processes
ii Automation and efficiency relies on digital components which requires dc
supply As public utilities supply ac power dc power supplies powered
by ac are needed by the dc loads
iii As more dc power supply are needed the converters that convert ac to dc
cause harmonics to be injected into the system and hence reduce wave
form quality
12 Problem Statement
With the increased use of sophisticated electronics high efficiency variable
speed drive and power electronic controller power quality has become an increasing
concern to utilities and customers Voltage sags is the most common type of power
quality disturbance in the distribution system It can be caused by fault in the electrical
network or by the starting of a large induction motor Although the electric utilities have
made a substantial amount of investment to improve the reliability of the network they
cannot control the external factor that causes the fault such as lightning or accumulation
of salt at a transmission tower located near to sea
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
4
Meanwhile during short circuits bus voltages throughout the supply network are
depressed severities of which are dependent of the distance from each bus to point
where the short circuit occurs After clearance of the fault by the protective system the
voltages return to their new steady state values Part of the circuit that is cleared will
suffer supply disruption or blackout Thus in general a short circuit will cause voltage
sags throughout the system but cause blackout to a small portion of the network [1]
A comprehensive study on the cost of losses due to power quality problem has
not been carried out yet However it has been reported that a petrochemical based
industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to
RM164000 (US$43000) per incident related to power quality problem due to voltage
sag Another semiconductor-based industry in the Klang Valley has estimated the loss of
RM5million for the year 2000 Other types of industries such the cement and garment
industries in Malaysia have also reported huge losses due power quality problems One
cement plant has reported an average loss of RM300 000 per incident [2]
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
5
Table 11 Cause of TNB network disruption [2]
In general voltage sags can causes
i Motor load to stallstop
ii Digital devices to reset causing loss of data
iii Equipment damage andor failure
iv Materials Spoilage
v Lost production due to downtime
vi Additional costs
vii Product reworks
viii Product quality impacts
ix Impacts on customer relations such as late delivery and lost of sales
x Cost of investigations into problem
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source with different type of loads
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
6
13 Project Objectives
The objectives of this project are
i To investigate suitable mitigation techniques for different type of voltage
sags source that connected to linear and non-linear load
ii To simulate and analyze the techniques using PSCADEMTDC software
iii To observe the effect on the characteristic of voltage sag such as the
magnitude and phase shift for each techniques
iv To make a few suggestions on the suitability of such techniques used for
both type of loads
14 Project Scope
The scopes for the project are
i Mitigation techniques that will be studied
a Dynamic Voltage Restorer (DVR)
b Distribution Static Compensator (D-STATCOM)
c Solid State Transfers Switch (SSTS) and
ii All techniques will be tested on different type of loads
iii Analysis will focus on effectiveness of each techniques in mitigating the
voltage sags
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
CHAPTER II
VOLTAGE SAGS
21 Introduction
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Tripping is caused by under voltage protection or over
current protection These two protections operate independently Large torque peaks
may cause damage to the shaft or equipment connected to the shaft Some common
reason for voltage sags are lightning strikes in power lines equipment failures
accidental contact power lines and electrical machine starts Despite being a short
duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place a small reduction in the system voltage can cause
serious consequences [5]
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
8
22 Definition of Voltage Sags
The definition of voltage sags is often set based on two parameters magnitude or
depth and duration However these parameters are interpreted differently by various
sources Other important parameters that describe voltage sags are
i the point-on-wave where the voltage sags occurs and
ii how the phase angle changes during the voltage sag A phase angle jump
during a fault is due to the change of the XR-ratio The phase angle jump
is a problem especially for power electronics using phase or zero-crossing
switching
The voltage sags as defined by IEEE Standard 1159 IEEE Recommended
Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current
at the power frequency for durations from 05 cycles to 1 minute reported as the
remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault
clearing times range from three to thirty cycles depending on the fault current magnitude
and the type of over current detection and interruption [4]
Terminology used to describe the magnitude of voltage sag is often confusing
The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which
means that line voltage is reduced to 20 of normal value Another definition as given
in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal
voltage for a time greater than 05 cycles of the power frequency but less than or equal
to 1 minute Usually further described using a modifier indicating the magnitude of a
voltage variation (eg sag swell or interruption) and possibly a modifier indicating the
duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21
shows the rectangular depiction of the voltage sag
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
9
Figure 21 Depiction of voltage sag
23 Standards Associated with Voltage Sags
Standards associated with voltage sags are intended to be used as reference
documents describing single components and systems in a power system Both the
manufacturers and the buyers use these standards to meet better power quality
requirements Manufactures develop products meeting the requirements of a standard
and buyers demand from the manufactures that the product comply with the standard
[2]
The most common standards dealing with power quality are the ones issued by
IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided
in next subtopic
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
10
231 IEEE Standard
The Technical Committees of the IEEE societies and the Standards Coordinating
Committees of IEEE Standards Board develop IEEE standards The IEEE standards
associated with voltage sags are given below [4]
IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power
systems for industrial and commercial applications range of sensibility loadsrdquo
The standard discusses the effect of voltage sags on sensitive equipment motor
starting etc It shows principles and examples on how systems shall be designed to
avoid voltage sags and other power quality problems when backup system operates
IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and
commercial power systemsrdquo
The standard proposes different techniques to predict voltage sag characteristics
magnitude duration and frequency There are mainly three areas of interest for voltage
sags The different areas can be summarized as follows [4]
i Calculating voltage sag magnitude by calculating voltage drop at critical
load with knowledge of the network impedance fault impedance and
location of fault
ii By studying protection equipment and fault clearing time it is possible to
estimate the duration of the voltage sag
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
11
iii Based on reliable data for the neighborhood and knowledge of the system
parameters an estimation of frequency of occurrence can be made
IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding
electronic equipmentrdquo
This standard presents different monitoring criteria for voltage sags and has a
chapter explaining the basics of voltage sags It also explains the background and
application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but
not as specific in defining different types of disturbances
IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power
qualityrdquo
The purpose of this standard is to describe how to interpret and monitor
electromagnetic phenomena properly It provides unique definitions for each type of
disturbance
IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary
voltage disturbancesrdquo
This standard describes the effect of voltage sags on computers and sensitive
equipment using solid-state power conversion The primary purpose is to help identify
potential problems It also aims to suggest methods for voltage sag sensitive devices to
operate safely during disturbances It tries to categorize the voltage-related problems that
can be fixed by the utility and those which have to be addressed by the user or
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
12
equipment designer The second goal is to help designers of equipment to better
understand the environment in which their devices will operate The standard explains
different causes of sags lists of examples of sensitive loads and offers solutions to the
problems [4]
232 Industry Standard
2321 SEMI
The SEMI International Standards Program is a service offered by
Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide
the semiconductor and flat panel display industries with standards and recommendations
to improve productivity and business SEMI standards are written documents in the form
of specifications guides test methods terminology and practices The standards are
voluntary technical agreements between equipment manufacturer and end-user The
standards ensure compatibility and interoperability of goods and services Considering
voltage sags two standards address the problem for the equipment [6]
SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage
sag immunityrdquo
The standard addresses specifications for semiconductor processing equipment
voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
13
is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a
voltage-duration graph shown in Figure 22
SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage
sag immunityrdquo
This standard defines a test methodology used to determine the susceptibility of
semiconductor processing equipment and how to qualify it against the specifications It
further describes test apparatus test set-up test procedure to determine the susceptibility
of semiconductor processing equipment and finally how to report and interpret the
results [6]
Figure 22 Immunity curve for semiconductor manufacturing equipment according
to SEMI F47 [6]
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
14
2322 CBEMA (ITI) Curve
Information Technology Industry (ITI formally known as the Computer amp
Business Equipment Manufactures Association CBEMA) is an organization with
members in the IT industry Within the organization the Technical Committee 3 (TC3)
has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC
input voltage that typically can be tolerated by most information technology equipment
The note is not intended to be a design specification (although it is often used by many
designers for that purpose) but a description of behavior for most IT equipment The
curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-
phase information technology equipment [IEEE 1100 ndash 1999]
The voltage-time curve in Figure 23 describes the border of an area Above the
border the equipment shall work properly and below it shall shutdown in a controlled
way
Figure 23 Revised CBEMA curve ITIC curve 1996 [7]
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
15
This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for
the following chapters The definitions provided by IEEE standards are the ones that are
used universally The characterization of voltage sags has also been discussed This
complies with the industry concerns related to the problem of power quality
24 General Causes and Effects of Voltage Sags
There are various causes of voltage sags in a power system Voltage sags can
caused by faults (more than 70 are weather related such as lightning) on the
transmission or distribution system or by switching of loads with large amounts of initial
starting or inrush current such as motors transformers and large dc power supply [3]
241 Voltage Sags due to Faults
Voltage sags due to faults can be critical to the operation of a power plant and
hence are of major concern Depending on the nature of the fault such as symmetrical or
unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal
respectively
For a fault in the transmission system customers do not experience interruption
since transmission systems are looped or networked Figure 24 shows voltage sag on all
three phases due to a cleared line-ground fault
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
16
Figure 24 Voltage sag due to a cleared line-ground fault
Factors affecting the sag magnitude due to faults at a certain point in the system
are
i Distance to the fault
ii Fault impedance
iii Type of fault
iv Pre-sag voltage level
v System configuration
a System impedance
b Transformer connections
The type of protective device used determines sag duration
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
17
242 Voltage Sags due to Motor Starting
Since induction motors are balanced 3 phase loads voltage sags due to their
starting are symmetrical Each phase draws approximately the same in-rush current The
magnitude of voltage sag depends on
i Characteristics of the induction motor
ii Strength of the system at the point where motor is connected
Figure 25 represents the shape of the voltage sag on the three phases (A B and
C) due to voltage sags
Figure 25 Voltage sag due to motor starting
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
18
243 Voltage Sags due to Transformer Energizing
The causes for voltage sags due to transformer energizing are
i Normal system operation which includes manual energizing of a
transformer
ii Reclosing actions
Figure 26 Voltage sag due to transformer energizing
The voltage sags are unsymmetrical in nature often depicted as a sudden drop in
system voltage followed by a slow recovery The main reason for transformer energizing
is the over-fluxing of the transformer core which leads to saturation Sometimes for
long duration voltage sags more transformers are driven into saturation This is called
Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
CHAPTER III
PSCADEMTDC SOFTWARE
31 Introduction
In this project all the mitigation technique PSCADEMTDC software will be
used to simulate and analyze the techniques Power System Aided Design (PSCAD) was
first conceptualized in 1988 and began its evolution as a tool to generate data files for
the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation
program In its early form Version was largely experimental Nevertheless it
represented a great leap forward in speed and productivity since users of EMTDC could
now draw their systems rather than creating text listings PSCAD was first introduced as
a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3
comes in 1994 bringing new usability by fully integrating the drafting and runtime
systems of its predecessors This integration produced an intuitive environment for both
design and simulation [15]
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
20
PSCAD Version 4 represents the latest developments in power system simulation
software With much of the simulation engine being fully mature form many years the
new challenges lie in the advancement of the design tools for the user Version 4 retains
the strong simulation models of it predecessors while bringing the table an updated and
fresh new look and feel to its windowing and plotting
32 Characteristics of Software
PSCAD is a powerful and flexible graphical user interface to the world-
renowned EMTDC solution engine PSCAD enables the user to schematically construct
a circuit run a simulation analyze the results and manage the data in a completely
integrated graphical environment Online plotting function controls and meters are also
included so that the user can alter system parameters during a simulation run and view
the results directly [15]
PSCAD comes complete with a library of pre-programmed and tested models
ranging from simple passive elements and control functions to more complex models
such as electric machines FACTS devices transmission lines and cables If a particular
model does not exist PSCAD provides the flexibility of building custom models either
by assembling them graphically using existing models or by utilizing an intuitively
Design Editor
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
21
The following are some common models found in systems studied using
PSCAD
i Resistors inductors capacitors
ii Mutually coupled windings such as transformers
iii Frequency dependent transmission lines and cables (including the most
accurate time domain line model in the world)
iv Current and voltage sources
v Switches and breakers
vi Protection and relaying
vii Diodes thyristors and GTOs
viii Analog and digital control functions
ix AC and DC machines exciters governors stabilizers and initial models
x Meters and measuring functions
xi Generic DC and AC controls
xii HVDC SVC and other FACTS controllers
xiii Wind source turbine and governors
PSCAD Version 4 has some major features that have been included prior to its
predecessors for usersrsquo convenience in modeling and analysis of custom power system
such as
i Windowing Interface ndash PSCAD V4 boasts a completely new windowing
interface which includes full MFC (Microsoft Foundation Class)
compatibility docking window support and a new integrated design
editor
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
22
ii Drawing Interface ndash the drawing interface has been enhanced to provide
uniform messaging and core support as well as a full double-buffered
display
iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have
been completely redesigned and are now more powerful The new
advanced graphs come complete with full features including full zoom
and panning support marker control Polymeter and XY plotting
capabilities
iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data
visualization and analysis software package available today PSCAD
output come to life
v Single-Line Diagram Input ndash PSCAD now includes the ability to
construct a circuits in a convenient and space saving single-line format
This new feature includes fully adaptive three-phase electrical
components in the Master Library can be adjusted easily to display a
single-line equivalent view
vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both
MATLABreg andor SIMULINKreg files
33 Example of Circuit
A typical DVR built in PSCAD and installed into a simple power system to
protect a sensitive load in a large radial distribution system [4] is presented in Figure 31
The coupling transformer with either a delta or wye connection on the DVR side is
installed on the line in front of the protected load Filters can be installed at the coupling
transformer to block high frequency harmonics caused by DC to AC conversion to
reduce distortion in the output The DC voltage source is an external source supplying
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
23
DC voltage to the inverter to convert to AC voltage The optimization of the DC source
can be determined during simulation with various scenarios of control schemes DVR
configurations performance requirements and voltage sags experienced at the point
DVR is installed
Figure 31 DVR with main components in PSCAD
The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge
Currents will follow in different directions at outputs depending on the control scheme
eventually supplying AC output power to the critical load during power disturbances
The control of this bridge is indeed the control of thyristor firing angles Time to open
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
24
and close gates will be determined by the control system There are several methods for
controlling the inverter To model a DVR protecting a sensitive load against only
balanced voltage sags a simple method of using the measurement of three-phase rms
output voltage for controlling signals can be applied Amplitude modulation (AM) is
then used In addition to provide appropriate firing angles to thyristor gates the
switching control using pulse width modulation (PWM) technique and interpolation
firing is employed
Figure 32 The Wye-Connected DVR in PSCAD
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
25
In Figure 32 the transformer is wye-connected with a common connection to the
midpoint of the DC source This allows that current will pump into each phase through
each pair of GTO and then return without affecting the other two phases It is noted that
to maintain an equal injecting voltage to each phase the same value of DC voltage at
each half of the source would be required
34 Conclusion
PSCAD Version 4 is a powerful tools to simulate and analysis custom power
systems With all the benefits designing a systems is as simple as using a drawing board
and a pencil in our hands Many new models have been added to the PSCAD Master
Library since the last release of PSCAD V3 thus improving capability of designing
Navigating the software is now has been made easy with the multi-window tab feature
and toolbars Common components were made available and easy to drag-and-drop it to
the drawing board
All those features were shadowed over with the limitation due to its commercial
value It has been described in the manual as Dimension Limits Those limits are divided
into two major groups which are Edition Specific Limits and Compiler Specific Limits
As for this project those limitations be of less interest because only one subsystem that
will be analysis for each mitigation technique
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
CHAPTER IV
VOLTAGE SAG MITIGATION TECHNIQUES
41 Introduction
Different power quality problems would require different solution It would be
very costly to decide on mitigate measure that do not or partially solve the problem
These costs include lost productivity labor costs for clean up and restart damaged
product reduced product quality delays in delivery and reduced customer satisfaction
Voltage sag can be classified in power quality problem Hence when a customer
or installation suffers from voltage sag there is a number of mitigation methods are
available to solve the problem These responsibilities are divided to three parts that
involves utility customer and equipment manufacturer Figure 41 shows the different
protection options for improving performance during power quality variation [1]
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
27
Figure 41 Different protection options for improving performance during power
quality variation [1]
This project intends to investigate mitigation technique that is suitable for
different type of voltage sags source with different type of loads The simulation will be
using PSCADEMTDC software The mitigation techniques that will be studied such as
using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)
and solid state transfer switch (SSTS)
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
28
42 Dynamic Voltage Restorer (DVR)
Voltage magnitude is one of the major factors that determine the quality of
power supply Loads at distribution level are usually subject to frequent voltage sags due
to various reasons Voltage sags are highly undesirable for some sensitive loads
especially in high-tech industries It is a challenging task to correct the voltage sag so
that the desired load voltage magnitude can be maintained during the voltage
disturbances [8]
The effect of voltage sag can be very expensive for the customer because it may
lead to production downtime and damage Voltage sag can be mitigated by voltage and
power injections into the distribution system using power electronics based devices
which are also known as custom power device [9] Different approaches have been
proposed to limit the cost causes by voltage sag One approach to address the voltage
sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag
at distribution level
441 Principles of DVR Operation
A DVR is a solid state power electronics switching device consisting of either
GTO or IGBT a capacitor bank as an energy storage device and injection transformers
It is connected in series between a distribution system and a load that shown in Figure
42 The basic idea of the DVR is to inject a controlled voltage generated by a forced
commuted converter in a series to the bus voltage by means of an injecting transformer
A DC capacitor bank which acts as an energy storage device provides a regulated dc
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
29
voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM
technique
During normal operating condition the DVR injects only a small voltage to
compensate for the voltage drop of the injection transformer and device losses
However when voltage sag occurs in the distribution system the DVR control system
calculates and synthesizes the voltage required to maintain output voltage to the load by
injecting a controlled voltage with a certain magnitude and phase angle into the
distribution system to the critical load [9]
Figure 42 Principle of DVR with a response time of less than one millisecond
Note that the DVR capable of generating or absorbing reactive power but the
active power injection of the device must be provided by an external energy source or
energy storage system The response time of DVD is very short and is limited by the
power electronics devices and the voltage sag detection time The expected response
time is about 25 milliseconds and which is much less than some of the traditional
methods of voltage correction such as tap-changing transformers [8]
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
30
43 Distribution Static Compensator (DSTATCOM)
In its most basic function the DSTATCOM configuration consist of a two level
voltage source converter (VSC) a dc energy storage device a coupling transformer
connected in shunt with the ac system and associated control circuit [10 11] as shown
in Figure 43 More sophisticated configurations use multipulse andor multilevel
configurations as discussed in [12] The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages These voltages are in phase and
coupled with the ac system through the reactance of the coupling transformer Suitable
adjustment of the phase and magnitude of the DSTATCOM output voltages allows
effective control of active and reactive power exchanges between the DSTATCOM and
the ac system
Figure 43 Schematic diagram of the DSTATCOM as a custom power controller
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
31
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes [13]
i Voltage regulation and compensation of reactive power
ii Correction of power factor
iii Elimination of current harmonics
The design approach of the control system determines the priorities and functions
developed in each case In this case DSTATCOM is used to regulate voltage at the point
of connection The control is based on sinusoidal PWM and only requires the
measurement of the rms voltage at the load point
441 Basic Configuration and Function of DSTATCOM
The DSTATCOM is a three phase and shunt connected power electronics based device
It is connected near the load at the distribution systems The major components of the
DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase
inverter module such as IGBT or thyristor ac filter coupling transformer and a control
strategy The basic electronic block of the DSTATCOM is the voltage sourced converter
that converts an input dc voltage into three phase output voltage at fundamental
frequency
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
32
Figure 44 Building blocks of DSTATCOM
Referring to Figure 44 the controller of the DSTATCOM is used to operate the
inverter in such a way that the phase angle between the inverter voltage and the line
voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the
desired VAR at the point of connection The phase of the output voltage of the thyristor
based converter Vi is controlled in the same way as the distribution system voltage Vs
Figure 45 shows the three basic operation modes of the DSTATCOM output current I
which varies depending upon Vi
For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM
does not generate or absorb reactive power When Vi is greater than Vs the
DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system
lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the
transformer reactance from the DSTATCOM to the ac system and the device generates
capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and
inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a
capacitive reactance then the current flows from the ac system to the DSTATCOM
resulting in the device absorbing inductive reactive power
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
33
Figure 45 Operation modes of a DSTATCOM
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
34
44 Solid State Transfer Switch (SSTS)
The SSTS can be used very effectively to protect sensitive loads against voltage
sags swells and other electrical disturbance [14] The SSTS ensures continuous high
quality power supply to sensitive loads by transferring within a time scale of
milliseconds the load from a faulted bus to a healthy one
The basic configuration of this device consists of two three phase solid state
switches one for main feeder and one for the backup feeder These switches have an
arrangement of back-to-back connected thyristors as illustrated in Figure 46
Figure 46 Schematic representations of the SSTS as a custom power device
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
35
Each time a fault condition is detected in the main feeder the control system
swaps the firing signals to the thyristor in both switches in example Switch 1 in the
main feeder is deactivated and Switch 2 in the backup feeder is activated The control
system measures the peak value of the voltage waveform at every half cycle and checks
whether or not it is within a prespecified range If it is outside limits an abnormal
condition is detected and the firing signals of the thyristors are changed to transfer the
load to the healthy feeder
441 Basic Configuration and Function of SSTS
The SSTS as shown in Figure 47 is a high speed open transition switch which
enables the transfer of electrical loads from one ac power source to another within a few
milliseconds
Figure 47 Solid State Transfer Switch system
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
36
The open-transition property of the SSTS means that the switch break contact
with one source before it makes contact with the other source The advantage of this
transfer scheme over the closed-transition mechanical switch is that the electrical
sources are never cross-connected unintentionally The cross connection of independent
ac sources with the alternate source switching on to a faulted system is discouraged by
electric utilities
The solid state transfer switch consists of two three phase ac thyristor switches
The thyristor operating in its two modes forms the key component of the SSTS In the
ON-state mode low impedance forward conduction of current takes place In the OFF-
state mode an open circuit with almost infinite impedance occurs in the thyristor
The basic ON-state and OFF-state properties of the thyristor are used to form an
intelligent switch which can choose between two upstream power sources providing the
better quality of supply available to the electrical load downstream The basic
configuration is based on anti-parallel thyristor group on preferred and alternate sides of
the switch A thyristor allows conduction only in forward direction Figure 48 illustrate
how the thyristors of transfer switch 1 can conduct either in the positive or the negative
half cycle of the ac sinusoid and the supply path is indicated by the bold line
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
37
Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle
of the preferred source
During normal operation thyristors associated with the preferred source are in
the ON-state normally closed (NC) position while those associated with the alternate
source are in the OFF-state normally open (NO) position
Current sensing circuits constantly monitor the states of the preferred and
alternate sources and feed the information to the monitoring high speed controller Upon
detecting the loss of the preferred source or voltage that is not within the preset range
the controller blocks the firing impulse signals to the gate-driven thyristors of transfer
switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe
interlocking mechanism Power then flows via the path as indicated by the bold line in
Figure 49
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
38
Figure 49 Thyristors on the alternate supply are turned ON on a sensing a
disturbance on the preferred source
The mechanical bypass equipment provides conventional transfer switch
functionality when the SSTS is in a thermal overload condition or is out of service for
testing or maintenance
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
CHAPTER V
MITIGATION TECNIQUES REALIZATION
51 Sinusoidal PWM-Based Control Scheme
In order to mitigate the simulated voltage sags in the test system of each
mitigation technique also to mitigate voltage sags in practical application a sinusoidal
PWM-based control scheme is implemented with reference to the DSTATCOM The
control scheme for the DVR follows the same principle The aim of the control scheme
is to maintain a constant voltage magnitude at the point where sensitive load is
connected under the system disturbance
The control system only measures the rms voltage at load point [10] in example
no reactive power measurements is required [17] The VSC switching strategy is based
on a sinusoidal PWM technique which offers simplicity and good response Since
custom power is a relatively low-power application PWM methods offer a more flexible
option than the fundamental frequency switching (FFS) methods favored in FACTS
applications Besides high switching frequencies can be used to improve the efficiency
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
40
of the converter without incurring significant switching losses Figure 51 shows the
DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM
control system exerts voltage angle control as follows an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point The PI
controller processes the error signal and generates the required angle δ to drive the error
to zero in example the load rms voltage is brought back to the reference voltage In the
PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle
δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared
against a triangular signal (carrier) in order to generate the switching signals of the VSC
valves
Figure 51 Control scheme for the test system implemented in PSCADEMTDC to
carry out the DSTATCOM and DVR simulations
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
41
The main parameters of the sinusoidal PWM scheme are the amplitude
modulation index ma of signal vcontrol and the frequency modulation index mf of the
triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC
The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental
voltage component at the controller output [13 18] The switching frequency mf is set at
450 Hz mf = 9 It should be noted that an assumption of balanced network and
operating conditions are made
The modulating angle δ or delta is applied to the PWM generators in phase A
whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively
It can be seen in Figure 51 that the control implementation is kept very simple by using
only voltage measurements as feedback variable in the control scheme The speed of
response and robustness of the control scheme are clearly shown in the test results
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
42
52 Test System
Figure 52 The test system implemented in PSCADEMTDC
Figure 52 depict the test system implemented in PSCADEMTDC to carry out
the simulations for the aforementioned mitigation techniques The test system comprises
of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent
feeding into the primary side of a 2-winding transformer The load is connected to the 11
kilovolt secondary side of the transformer Another 3-winding transformer will be used
to replace the 2-winding transformer to accommodate the implantation of the two-level
DSTATCOM and it will be connected in the tertiary winding of the transformer to
provide instantaneous voltage support at the load point The transformer employ a
leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster
capabilities exist
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
43
53 Dynamic Voltage Restorer
The DVR is a powerful controller that is commonly used for voltage sags
mitigation at the point of connection The DVR employs the same block as the
DSTATCOM but in this application the coupling transformer is connected in series with
the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output
voltage which is controllable in phase and magnitude These voltages are injected into
the ac system in order to maintain the load voltage at the desired voltage reference The
main features of the DVR control scheme have been explained in section 51
Figure 53 One line diagram of the DVR test system
The DVR that have been used to test the system in section 51 is shown in Figure
54 The DVR is basically the same as DSTATCOM but instead of using a capacitor
DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using
transformers in delta to the lines Figure 55 will show the full test system to realize the
effectiveness of the DVR control
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
44
Figure 54 Schematic diagram of the DVR
Figure 55 Schematic diagram of the test system with DVR connected to the system
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
45
54 Distribution Static Compensator
The test system employed to carry out the simulations concerning the
DSTATCOM actuation is shown in Figure 29 which is the same system presented in
[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point A 750 microF capacitor on the dc side
provides the DSTATCOM energy storage capabilities
The transformer of the test system has been changed to a 3-winding transformer
to accommodate DSTATCOM The purpose of including the transformer is to protect
and provide isolation between the IGBT legs This prevents the dc storage capacitor
from being shorted through switches in different IGBT Figure 56 shows the build of
the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter
and the realization of the test system being employed shown in Figure 57
Figure 56 One line diagram of the DSTATCOM test system
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
46
Figure 57 Schematic diagram of the test system with DSTATCOM connected to the
system
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
47
55 Solid State Transfer Switch
In the test to carry out the SSTS simulations the system comprises with two
identical feeders from section 51 and a sensitive load connected to the bus bar Figure
58 shows the system that is employed
Figure 58 One line diagram of the SSTS test system
Simulations were carried out to assess the effectiveness of the simple control
scheme that has been employed in the system proposed earlier Figure 59 shows the
SSTS system that being employed for the test in PSCADEMTDC It comprises of two
sets of switches which is switch group 1 and switch group 2 that alternately turns ON
and OFF corresponds to the fault detector signals The full system application to test the
SSTS is shown in Figure 510
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
48
Figure 59 SSTS switches implemented in PSCADEMTDC
Figure 510 Schematic diagram of the test system with SSTS connected to the system
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
CHAPTER VI
SIMULATIONS AND RESULTS
61 Test case
This section contains the results of the simulations to assess the capability of
each technique to mitigate various fault sources In order to make a fair assessment the
simulations only use one test system as proposed in section 51 The test were divide into
the most common faults which are
611 Single line to ground fault and
612 Double line to ground fault
The most common fault is the single line to ground faults which covers 70 of
total faults There are many situations that can make the occurrence of single line to
ground faults possible The low impedance faults are referred to as bolted faults
indicating that the faulted conductors are effectively bolted together to create a line to
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
50
line faults which cover 10 of the total faults or double line to fault for the total of 15
A much more common effect is where the fault has some finite impedance When a line
falls on sandy soil or there is a significant distance for an arc to jump then the
characteristic may have a constant voltage characteristic The remaining 5 of the faults
are three phase faults
62 Single line to ground fault
621 Phase A to ground
Using the faults generator Figure 61a clearly shows a phase shift of line A after
the fault has been applied The angle of the line shifted as much as 8844deg from the
reference angle for line A of -194deg For the rms value of the line we can refer to Figure
61b which clearly shows the voltage sag The value of the rms has been normalized and
for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the
reference value
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
51
(a)
(b)
Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop
The simulations have two parts which have been run separately This first part
involves simulating the test system on different fault as mention above The second part
involves simulating the mitigation techniques with the test system so that each of the
technique can be assessed on their performance in mitigating voltage sags
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
52
(a)
(b)
Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR
The first technique that has been used is the DVR Figure 62a shows the
capability of the technique to balance the phase shift while Figure 62b shows how the
technique compensates the voltage drop DVR recover almost 96 of the reference
voltage
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
53
The second technique that has been used in mitigating the voltage sags and phase
shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure
63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly
94 of the voltage with respect to the reference voltage
(a)
(b)
Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag
using DSTATCOM
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
54
The third technique that has been used is SSTS In SSTS whenever the fault
detector control scheme detects a faulty line it changes the firing angle of the switches
that are connected to the line thus change the feed from the main feeder to the alternative
or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be
noticed since the backup feeder is healthy
(a)
(b)
Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using
SSTS
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
55
Since SSTS switch the faulty feeder with the healthy one whenever faults occur
as long as the back up feeder is healthy the result produced by this technique will
always be the same Hence the result of the SSTS will be omitted hereafter with the
assumption that the backup feeder is always healthy
Table 61 (a) Test results for line A to the ground fault (b) Recovery result
TEST 1 PHASE A TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12194 11806 0685 0991
DVR 075 -9893 9832 0923 0963
DSTATCOM 128 -14787 1424 0948 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 1 PHASE A TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 8963 2301 1974 9585
DSTATCOM 891 2593 2434 9377
SSTS 8849 005 005 100
(b)
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
56
From table 61a and 61b we can see that SSTS has the best recovery rate since it
doesnrsquot involve compensating technique either to absorb or inject power to the system
The rms value of the system is always constant It is different than the other two
techniques which require them to inject or absorb power to and from the system DVR
has better recovery in mitigating the voltage sag than DSTATCOM but poor in
correcting the phase of the lines DVR recover 2 better in comparison with
DSTATCOM
622 Phase B to ground
For test 2 the faults generator still emulates a single line to ground fault of line
B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty
system is as the same as Figure 61b The only difference is in the phase of the system
Figure 65 show the shifted phase of the system when the fault occurs
Figure 65 Phase shift of line B to the ground fault
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
57
It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the
fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the
result for DSTATCOM for phase correction Each technique recovers the same value of
the rms as when it mitigates the phase A to the ground fault
(a)
(b)
Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B to the ground fault
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
58
From the figure above it can be observed that other line phases were also
affected when both techniques try to correct the lines phase The effect can be clearly
noted in Figure 66a where the phase of line A and C are shifted even though those lines
were not in fault This condition as well happen when DSTATCOM try to correct the
phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show
the recoveries that have been achieved by those three techniques
Table 62 (a) Test results for line B to the ground fault (b) Recovery result
TEST 2 PHASE B TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 14964 11806 0686 0991
DVR -21 -11856 140 0923 0963
DSTATCOM 1583 -12237 9672 0942 1016
SSTS -189 -12189 11811 0989 0989
(a)
TEST 2 PHASE B TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1906 3108 2194 9585
DSTATCOM 1389 2727 2134 9272
SSTS 005 2775 005 100
(b)
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
59
DVR manage to recover 9585 of the rms voltage with respect to the reference
value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always
100 since the backup feeder is healthy
623 Phase C to ground
Test 3 involves line C of the system This test is practically the same as previous
test which only involves 1 line of the system The results of the rms voltage is the same
as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in
Figure 67
Figure 67 Phase shift of line B to the ground fault
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
60
Mitigation of the fault outcome is the same product as the preceding test which
DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure
68(b) shows the phase difference for the mitigation technique accordingly
(a)
(b)
Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line C to the ground fault
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
61
The numerical result will be shown in Table 63(a) whereas the recovery will be
shown in Table 63(b) The phase of line C has been corrected but at the same time
other lines were also affected This is true for both of the technique but not for SSTS
which is the same as Figure 64(a) and Figure 64(b)
Table 63 (a) Test results for line C to the ground fault (b) Recovery result
TEST 3 PHASE C TO GROUND
PHASE(deg) RMS(pu) TECHNIQUES
A B C min max
FAULT -194 -12194 2969 0686 0991
DVR 1969 -13945 11742 0923 0963
DSTATCOM -2283 -10183 12867 0914 1011
SSTS -189 -12189 11811 0989 0989
(a)
TEST 3 PHASE C TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 1775 1751 8773 9585
DSTATCOM 2089 2011 9898 9041
SSTS 005 005 8842 100
(b)
From the table line A and line B should have stay fixed on 0deg and -120deg
respectively but after DVR and DSTATCOM try to correct the phase of line C the
phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for
DSTATCOM This could be due to the control scheme that is too simple In the mean
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
62
time the rms voltage compensation for both DVR and DSTATCOM are still above 90
in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage
This is true for the entire tests that have been carried out before while SSTS results are
overwhelming with no ripple or overshoot
63 Double lines to ground fault
The next line of test is double line to the ground fault As an overall those
techniques except SSTS suffer terrible loss when its try to mitigate double line to the
ground fault This fault only covers 15 of overall fault that occurs practically but it
pose much more danger to the loads that draw supply from the lines
631 Phase A and B to ground
The first test to come is line A and line B to the ground fault The effect of this
fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that
shows the rms voltage of the test system during the fault
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
63
(a)
(b)
Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop
For this test the phase A and B has been shifted 90deg to -90deg and 150deg
respectively The voltage drop is doubled from previous test set to 0366 per unit with
respect to the reference voltage Figure 610(a) shows the result of the DVR try to
correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
64
(a)
(b)
Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and B to the ground fault
As we can see from the figure DVR continue to correct the phases of the faulted
lines steadily with almost the same value at the time DVR is correcting the single line to
ground fault The same abnormality happens with the line that doesnrsquot need any
correction and in this case it is line C The phase of line C is shifted nearly 10deg
However DSTATCOM capability of correcting the phase of single line to the ground
fault has not been continual for the double line to the ground fault For lines A and B to
the ground fault DSTATCOM is able to correct the phase of line B but this is not
occurred to line A The phase is shifted about 140deg and rest at 50deg
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
65
Even though the voltage sag is double from the previous value DVR manage to
compensate the voltage drop and recovered nearly 90 with respect to the reference
voltage DSTATCOM only manage to recover 78 This is due to the inability of
DSTATCOM to mitigate double line to the ground fault with only using simple control
scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and
611(b) for DVR and DSTATCOM respectively
(a)
(b)
Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag
using DSTATCOM Line A and B to the ground fault
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
66
The value of voltage sag that have been recovered for other double lines to the
ground fault such as line A and C to the ground fault and line B and C to the ground
fault is the same as the result shown in Figure 611 Hence those results are omitted
hereafter
Table 64(a) will show the full result of line A and B to the ground fault while
Table 64(b) shows the recovered voltage sag and corrected phase for those lines
Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result
TEST 4 PHASE AB TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 14966 11806 0366 0991
DVR -078 -1106 110331 0858 0963
DSTATCOM 4961 -12336 11725 0777 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 4 PHASE AB TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 896 3906 7729 891
DSTATCOM 4077 263 081 7841
SSTS 8849 2777 005 100
(b)
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
67
632 Phase A and C to ground
The next test case is line A and C to the ground fault As mention before the
result of voltage sag that is mitigated is the same as the result for section 631 DVR and
DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the
results of voltage sag mitigation of this section are omitted
Figure 612 Phase shift for line A and C to the ground fault
Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg
to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the
fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for
DVR and DSTATCOM respectively
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
68
(a)
(b)
Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line A and C to the ground fault
The result in Figure 613(b) clearly shows the improper phase correction of line
C which definitely affect the result of DSTATCOM voltage mitigation while in Figure
613(a) DVR also cannot correct the phase accurately The full test result is shown in
Table 65(a) while Table 65(b) shows the recovery result
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
69
Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result
TEST 5 PHASE AC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -9038 -12193 2965 0365 0991
DVR -1982 -11938 1393 0858 0963
DSTATCOM 286 -12898 17872 0769 0995
SSTS -189 -12189 11811 0989 0989
(a)
TEST 5 PHASE AC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 7056 255 10965 891
DSTATCOM 8752 705 14907 7729
SSTS 8849 004 8846 100
(b)
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
70
633 Phase B and C to ground
The last test case is line B and C to the ground fault In this case phase B is
shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively
This can be seen in Figure 614 as it shows the phase shift of the faulty lines
Figure 614 Phase shift for line B and C to the ground fault
The phase of line A is unaffected by the fault of other lines throughout the fault
period However the phase of the line is affected and shifted 30deg for the moment of
mitigation using DVR This affect is obviously depicted in Figure 615(a)
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
71
(a)
(b)
Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM
line B and C to the ground fault
As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)
is not corrected appropriately and this time it is line B The phase of the line at the time
of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in
Table 66(a) and the recovery result is shown in Table 66(b)
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
72
Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result
TEST 6 PHASE BC TO GROUND
PHASE(deg) VRMS(pu) TECHNIQUES
A B C min max
FAULT -193 14965 2968 0365 0991
DVR 3073 -13593 14793 0858 0963
DSTATCOM -626 -616 12603 0768 0991
SSTS -189 -12189 11811 0989 0989
(a)
TEST 6 PHASE BC TO GROUND RECOVERY
PHASE(deg) VRMS() TECHNIQUES
A B C GAIN
DVR 288 1372 11825 891
DSTATCOM 433 8805 9635 775
SSTS 004 2776 8843 100
(b)
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
73
64 Conclusion
In mitigating single line to the ground fault DVR and DSTATCOM that has
been introduced in section 5 are able to compensate the voltage sag without any
difficulty The problem lies in correcting the phase of the system Even though the phase
of the faulty line has been corrected the rest of the lines that are not in fault is also
affected and shifted a few degrees This affect can be seen happened to DVR when it
mitigates the test system In general the capability of the techniques to mitigate single
line to the ground fault are uncontested especially SSTS as it pose the best result
While mitigating double lines to the ground fault the same problems occurred to
the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the
performance of DVR in mitigating voltage sag remain the same as it mitigates single
line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM
is mitigating double line to the ground fault One of the faulty lines is not corrected
appropriately and this brings an upsetting effect in mitigating the voltage sag of the
system Once again SSTS that has been introduced in section 5 remain as the best
mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to
compensate or correct the faulty line instead SSTS switch the faulty feeder to the
alternative feeder The result is always and remains constant if and only if the backup or
alternative feeder is being kept healthy
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
CHAPTER VII
CONCLUSION
71 Conclusion
Nowadays reliability and quality of electric power is one of the most discuss
topics in power industry There are numerous types of power quality issues and power
problems and each of them might have varying and diverse causes The types of power
quality problems that a customer may encounter classified depending on how the voltage
waveform is being distorted There are transients short duration variations (sags swells
and interruption) long duration variations (sustained interruptions under voltages over
voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics
notching and noise) voltage fluctuations and power frequency variations Among them
two power quality problems have been identified to be of major concern to the
customers are voltage sags and harmonics but this project is focusing on voltage sags
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
75
Voltage sags are huge problems for many industries and it is probably the most
pressing power quality problem today Voltage sags may cause tripping and large torque
peaks in electrical machines Generally voltage sags are short duration reductions in rms
voltage caused by faults in the electric supply system and the starting of large loads
such as motors Voltage sags are also generally created on the electric system when
faults occur due to lightning which are accidental shorting of the phases by trees
animals birds human error such as digging underground lines or automobiles hitting
electric poles and failure of electrical equipment Sags also may be produced when large
motor loads are started or due to operation of certain types of electrical equipment such
as welders arc furnaces smelters etc
Therefore this project intends to investigate mitigation technique that is suitable
for different type of voltage sags source The simulation will be using PSCADEMTDC
software and the mitigation techniques that using such as dynamic voltage restorer
(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch
(SSTS)
Dynamic voltage restorers (DVR) are used to protect sensitive loads from the
effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR
control system to not only detect the start and end of a voltage sag but also to determine
the sag depth and any associated phase shift The DVR which is placed in series with a
sensitive load must be able to respond quickly to voltage sag if end users of sensitive
equipment are to experience no voltage sags
The distribution static compensator (DSTATCOM) offers an alternative to
conventional series shunt compensation In the traditional power transmission system
controllable devices are restricted to the slow mechanisms such as transformer tap
changers and switched capacitor In the late 1980rsquos thanks to the major developments
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
76
in the semiconductor technology it became possible to apply power electronics in the
control of DSTATCOM Based on the simulation therersquos a room for improvement
DSTATCOM is a device that promises a prominent feature in power system in
mitigating power quality related problems in the future
Solid state transfer switch (SSTS) is not the most cost effective but in many
cases it is a practical mitigating technique to apply especially for sensitive loads These
solutions involve fixing the two identical power source components in order to increase
the ride-through of the entire system SSTS solutions are attractive since they in theory
do not require add on power conditioning equipment but instead involve using another
source components Furthermore semiconductor tool suppliers are more comfortable
with this approach since it does not require the addition of unfamiliar technologies
As conclusion voltage sag is unwanted phenomenon which unavoidable but can
be reduced using all techniques but not limited to the techniques that have been
discussed There is no one mitigation technique that will suitable with every application
and whilst the power supply utilities strive to supply improved power quality it is up to
the applications engineer to minimize power quality problems It means power quality
problem cannot be eliminated but we can reduce and try to avoid this problem form
occur The best way to avoid power quality problem is by ensuring that all equipment to
be installed in the industrial plants are compatible with power quality in the power
system This can be achieved by procuring equipment with proper technical
specifications that incorporate power quality performance of its operating electrical
environment
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
77
72 Suggestion
Mitigating voltage sag requires a lot of intensive research especially in
developing custom power device to help distribution system to achieve desired power
quality as been insisted by many customer or end-user There are still rooms of
improvement that can be achieved further for the technique that have been included in
this thesis and other techniques that are available
The DVR and DSTATCOM that has been used earlier employs a two- level
voltage source converter or VSC in both technique Additional research of other
multilevel and multipulse VSC can be implemented in the future to exploit the simplicity
of the pulse width modulation or PWM based control scheme to further enhance both
DVR and DSTATCOM Another control scheme can also be proposed to take the
advantage of the two-level VSC that has been employed previously to support more
control over voltage sags that were caused by double line to ground line to line faults
and three phase fault that cover 25 percent of the total faults
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
78
REFERENCES
[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty
TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages
1-8 and 39-80
[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic
In Power Engineering session 20052006-II
[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-
Monitoring Analysis amp Mitigationsrdquo pages 1-61
[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE
Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New
York
[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage
sagsrdquo available at httpwwwieeeorgias
[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at
httpwwwsemiorg
[7] ldquoITI (CBEMA) curve application noterdquo Available at
httpwwwiticorgtechnicaliticurvpdf
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
79
[8] M H Haque (2001) Compensation of Distribution System Voltage Sag
by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001
[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-
Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference
on Research and Development PROCEEDINGS Shah Alam Malaysia
[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe
implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power
Eng Rev pp 61-62 Dec 1998
[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of
custom power devices for power quality enhancementrdquo in Proc 9th ICHQP
2000 Orlando FL Oct 2000 pp 775-783
[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of
multilevel converters under multiple regulation feedback controlrdquo IEEE Trans
Power Electron vol 14 pp 959-965 Sept 1999
[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic
Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann
2001
[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with
solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct
1998 pp 210-215
[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos
Tool for Power System Simulation
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
80
[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power
systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17
(1) pp 266-272 2002
[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of
conventional and advanced static var compensators in electromagnetic transients
simulation programrdquo Available at httpwwweeumanitobaca~hvdc
[18] N Mohan T M Underland and W P Robbins ldquoPower electronics
Converters Application and Designrdquo New York Wiley 1995
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
81
APPENDIX A
Data generated by PSCADEMTDC for DSTATCOM
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
82
14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
83
APPENDIX B
Data generated by PSCADEMTDC for DVR
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
84
--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
85
APPENDIX C
Data generated by PSCADEMTDC for SSTS
======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0
86
DATADSD DATADSO ENDPAGE
86
DATADSD DATADSO ENDPAGE