RESTRICTED

organisation where research was done)*

OPEN ACCESS

:

I acknowledged that UniversitiTeknologi Malaysia reserves the right as follows:

: ABDIFATAH MOHAMED MOHAMUD

AHMED SALAD OSMAN

ABUBAKAR HASSAN TAKOW

DECLARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND COPYRIGHT

UNIVERSITY OF HORMUUD

Author’s full name

: ENHANCEMENT OF POWER QUAITY IN

DISTRIBUTION SYSTEM BY D-STATCOM

Academic Session 2014/2015

I declare that this thesis is classified as:

CONFIDENTIAL (Contains confidential information)

(Contains restricted information

I agree that my thesis to be published as online open access

( full text)

1. The thesis is the property of University of Hormuud.

2. The Library of University of Hormuud has the right to make copies for the

purpose of research only.

3. The Library has the right to make copies of the thesis for academic exchange.

NOTES : If the thesis is CONFIDENTIAL or RESTRICTED, please attach with the letter from

The organisation with period and reasons for confidentiality or restriction.

Title

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as by the

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(Contains restricted information as specified by the

organization where research was done)

We acknowledged that University of Hormuud reserves the right as follows

SIGNATURE OF SUPERVISOR

SAID OSMAN MOHAMMED

NAME OF SUPERVISOR

DATE: 1 AUGUST 2015

Certified by:

We declare that this thesis is classified as:

Author’s full names

Title ENHANCEMENT OF POWER QUALITY IN DISTRIBUTION

SYSTEM BY USING D-STATOCM

Academic year

ENHANCEMENT OF POWER QUALITY IN DISTRIBUTION SYSTEM BY

USING DISTRIBUTION STATIC DCOMPENSATOR (D-STATCOM)

Ahmed Salad Osman

Abdifatah Mohammed Mohammud

Abubakar Hassan Takow

A report submitted in fulfillment of the requirements for the award of the degree of

Bachelor of Engineering (Electrical)

FACULTY OF ELECTRICAL ENGINEERING

UNIVERSITY OF HORMUUD

AUGAST 2015

“I declare that I have read this project and in my opinion this project report is

adequate in term of scope and quality for the purpose of awarding a Bachelor‘s

degree of Electrical Engineering”

External examiners

MOHAMUD FARAH ALI :

ALI ADDAWE GEDI :

Signature

Signature :

Name of Supervisor : SAID OSMAN MOHAMMED

Date : AUGUST 1 2015

II

“We hereby declared that the following thesis entitled ‘power quality

enhancement in distribution system Using Distribution Static Compensator (D-

STATCOM)’ is the result of our own effort except as cited in the references”

Signature :

Name : ABDIFATAH MOHAMMED MOHAMMUD

Signature :

Name : AHMED SALAD OSMAN

Name : ABUBAKAR HASSAN TAKOW

Signature :

Date : AUGUST 1 2015

III

ACKNOWLEDGMENT

First of all we thank Allah SWT, who gave us the opportunity and strength to

carry out this project.

We would like to express our sincere appreciation to our final year project

supervisor, Eng Said Osman Mohamed for encouragement, guidance, critics and give

US a lot of motivation in order to complete this project.

We also want to express our warm thanks to my friends who supported us in

this work for all their help, support, interest and valuable hints. We really appreciate

it and will forever be indebted to them.

Lots of thanks to our beloved mothers and fathers, for their love and support

and always stay beside us and always pray for us to success in study.

IV

ABSTRACT

Nowadays, the term power quality has becoming increasingly concerned by

both electric utilities and end users of electrical power. Power quality problems such

as transient, short duration variations (sags, swells and interruption), voltage

imbalance, waveform distortion (dc offset, harmonics, inter harmonics, notching and

noise), voltage fluctuations and power frequency variations can affect the

performance of the equipment at consumer. The most affected due to these problems

is industrial customers who use a lot of sensitive equipment. They have suffered a

huge loss due to this problem. Thus, device such as Static Synchronous Compensator

(STATCOM), Dynamic Voltage Restorer (DVR) and Uninterruptable Supply (UPS)

has been created to solve this problem. D-STATCOM has it respective limitations

such as capabilities and functions. The project will focus on how the D-STATCOM

can regulate the voltage in order to improve the voltage sag in the system. A

simulation was carried out using MATLAB/Simulink software to obtain the result.

When there was an overload in the system, high current will flow through the line

and the consequence voltage sag will occur. In order to improve voltage sag, D-

STATCOM was connected to the system. In this project, the performance of D-

STATCOM both before and after saging will be analyzed.

V

TABLE OF CONTENTS

Chapter Title Page

Declaration

Dedication Ii

Acknowledgment Iii

Abstract Iv

Table of contents V

List of tables Viii

List of figures Ix

List of symbols Xi

List of abbreviations Xii

List of appendices Xiii

1 INTRODUCTION

1.1 introduction 1

1.2 problem statement 3

1.3 objectives 4

1.4 scope 4

1.5 structure of the thesis 5

2 LITERATURE REVIEW

2.1 introduction 6

2.2 power quality problems and solutions 7

2.2.1 introduction 7

2.2.2 power quality problems 8

2.2.2.1 interruption 8

2.2.2.2 sags 9

VI

2.2.2.3 swells 10

2.2.2.4 harmonics 11

2.2.2.5 harmonic distortion 12

2.2.2.6 harmonic source from commercial loads 13

2.2.2.7 harmonic source from industrial loads 13

2.2.2.8 voltage spikes 14

2.2.2.9 voltage unbalance 14

2.2.3 sources of power quality problems 15

2.2.3.1 power electronic devices 15

2.2.3.2 IT and office equipment 15

2.2.3.3 arcing device 16

2.2.3.4 load switching 16

2.2.3.5 large motor starting 17

2.2.3.6 sensitive equipments’ 17

2.2.3.7 storm and environment related damage 17

2.2.3.8 capacitor switching 18

2.2.4 power quality improvement techniques 18

2.3 voltage sag 19

2.3.1 sources of voltage sag 19

2.3.2 factors of voltage sag 20

2.3.3 types of volts that cause voltage sag 21

2.3.3.1 single line to ground fault 22

2.3.3.2 double line to ground fault 22

2.3.3.2 three phase fault 22

2.3.4 impacts of voltage sag 23

2.4 custom power devices 25

2.4.1 Dynamic Voltage Restorer (DVR) 25

2.4.1.1 introduction 25

2.4.1.2 DVR components 26

` 2.4.1.3 operating principle of DVR 29

2.4.2 unified power quality conditioner (UPQC) 30

2.4.2.1 introduction 30

VII

2.4.2.1 classification of UPQC and operating principle 31

2.4.3 distribution static compensator (D-STATCOM) 33

2.4.3.1 introduction 33

2.4.3.2 main components of a D-STATCOM 35

2.4.3.3 distribution static compensator configuration 37

2.4.3.4 D-STATCOM V-I characteristic 38

2.4.3.5 basic configuration and function of D-STATCOM 40

2.4.3.6 calculation of voltage injection by D-STATCOM 42

2.4.3.7 operation modes of a D-STATCOM 43

3 METHODOLOGY

3.1 introduction 46

3.2 project flow 46

3.3 simulations 48

3.3.1 system implementation in MATLAB/simulink 50

3.4 conclusion 57

4 RESULT AND DISCUSSION

4.1 introduction 58

4.2 simulations without insertion D-STATCOM scheme 58

4.3 Simulations with installation D-STATCOM system 63

4.4 conclusion 67

5 CONCLUSION AND RECOMMENDATION

5.1 conclusion 69

5.2 recommendations 70

5.3 references 71

5.4 Appendix A 75

Appendix B 76

Appendix C 77

VIII

LIST OF TABLES

Table Title Page

4.1 Results of voltage sags for different types of faults 62

4.2 Results of voltage sags after compensation for different types of

faults

63

IX

LIST OF FIGURES

Figure Title Page

2.1 Short interruptions 8

2.2 Long interruptions 9

2.3 Voltage sag or dip 10

2.4 Voltage swell 11

2.5 Harmonics 12

2.6 Voltage spike 14

2.7 Voltage unbalance 15

2.8 Disturbance caused by energizations capacitor bank 18

2.9 Dynamic Voltage Restorer( DVR) scheme diagram 26

2.10 Filter placed in high voltage side 28

2.11 Filter placed in low voltage side 28

2.12 Schematic diagram of DVR based on compensation system 29

2.13 Single line representation of conventional UPQC 32

2.14 General arrangement of STATCOM 35

2.15 Schematic diagram of STATCOM 38

2.16 D-STATCOM V-I characteristic 39

2.17 Schematic diagram of D-STATCOM 42

2.18 No-load mode (Vs=Vi) 44

2.19 Capacitive mode (Vi>Vs) 44

2.20 Inductive mode (Vs>vi) 45

X

3.1 Flow chart of project process 47

3.2 Flow chart of simulation process 49

3.3 The main circuit implemented in MARLAB/simulink 51

3.4 Sample run time of simulation 51

3.5 Configuration for fault generator 52

3.6 Energy storage and voltage source converter 53

3.7 Configuration of voltage source converter 54

3.8 Control scheme design for the D-STATCOM 54

3.9 Control mechanism 55

3.10 D-STATCOM system connected in distribution system 56

4.1 Main circuit of distribution system without D-STATCOM 59

4.2 Reference signal before three phase fault applied 59

4.3 Reference signal for each phase A,B and C before fault respectively 59

4.4 Reference signal after the three phase fault applied of voltage sag 61

4.5 Voltage output at feeder A for each phase A,B and C respectively 61

4.6 Voltage at load point for TLG,DLG and SLG without D-STATCOM 63

4.7 Main circuit of distribution system with D-STATCOM 64

4.8 Three phase output voltage after compensation 64

4.9 Output voltage at feeder A after compensation for each phase A,B.

and C respectively

65

4.10 Voltage at load point for TLG,DLG and SLG with D-STATCOM 66

XI

LIST OF SYMBOLS

R Resistance

L Inductance

Zth Thevenin equivalent

Vi Output voltage

Vs Source voltage

Ish Shunt current

Il Load current

IS Source current

VTH Thevinin voltage

ƞ Line angle

α Thevenin voltage angle

ß Thevenin impedance angle

VL Load voltage

Ssh Complex power exchange

XII

LIST OF ABBREVIATIONS

FACTS Flexible ac transmission systems

STATCOM Static compensator

D-STATCOM Distribution static compensator

IEEE Institution of electrical engineering

RMS Root mean square

SLG Single line to ground

DLG Double line to ground

TLG Triple line to ground

SVC Static VAR compensator

VSC Voltage source converter

PWM Pulse width modulator

Vi Output voltage

Vs Source voltage

DC Direct current

AC Alternating current

PI Proportional integral

PLC Programmable logic controllers

DVR Dynamic voltage restorer

UPQC Unified power quality conditioner

IGBT Insulated gate bipolar transistor

GTO Gate thyristor turn off

XIII

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Configuration and specification

B The main circuit of distribution system without D-

STATCOM system

C The main circuit of the distribution system with D-

STATCOM system

1

CHAPTER ONE

INTRODUCTION

1.1 Introduction

Our country Somalia is low power quality which caused different problems,

one of those problems we fix in our thesis project, knowing that the cost of electricity

in Somalia is very high that caused by lack of power quality so our thesis will

concentrate how to enhance the power quality , therefore we require high power

quality , reliable electrical power and rising number of distorting loads may leads to

an increased awareness of power quality both by clients and utilities , the mainly

general power quality problems today are voltage sags, harmonic distortion and low

power factor .

Power quality is one of the most important aspect concerned by utility and

residential customer, especially when it becomes a very sensitive issue for industrial

consumer [1]. Power quality means the ability to receive pure electrical voltage

sinusoidal waveform at delivery point [1]. It is obvious that everyone will aim to gain

the excellent quality in every situation no matter in which aspect, and power quality

should not be excluded. Power quality problems are not new issues in a electrical

power system and every electrical user might want to minimize their effect on

electrical equipment so that the highest possible power quality could be obtained by

all of the users [2].

2

The changes of the voltage supplied even for very short period of time, which

were not really mostly taken attention by public, is now very expensive due to their

cause of improper operation and shut down situation in manufacturing plants. For

the purpose of getting the highest efficiency in production besides for sustaining of

the most reasonable operating cost, electrical customers were now eager for the high

power quality. For instance, disturbances like voltage sag, which was introduced by

the higher fault on the network, will influenced more number of customer victims

[1]. Therefore, a proper study and determination about the power quality

disturbances should be conducted seriously as well as the extenuation manners not

only to fulfill customers demand, but also increase the reputation and quality of

electrical power in our country.

In order to further understand the sophisticated power quality problems, the

recordings of all disturbances are now done by installing the on-line power quality

monitoring system. Besides that, various professional surveys had conducted in date

[1]. Hence, it is obviously that power quality disturbances are now became the very

crucial topic for us to understand and study more. It means that the detection and

mitigation of voltage sag in electrical power system are also very important to

achieve improvement in power system so that their benefits could be experienced by

people in whole country.

From decades to decades, power electronics have been introduced and

developed further due to its economical and power saving advantages. Flexible AC

Transmission System (FACTS) are widely used to solve power quality disturbances

and Distribution Static Compensator (DSTATCOM) is one of the members of

FACTS devices family which is effective and flexible. Its function is similar to the

usage of synchronous transformer. In other words, DSTATCOM is a fast-respond

reactive power source compensator, which can properly solve varies power

disturbances with appropriate controller designed, such as voltage sag, voltage swell,

flicker, harmonic, and transient. It contains an injection transformer, a voltage source

converter (VSC) and a PWM controller with specific control scheme in order to

perform its main function efficient and effectively. In this thesis, the function of

3

DSTATCOM in voltage mitigation was mainly be discussed and it is one of the most

important function of D-STATCOM devices.

1.2 Problem statement

Electronic equipment have been evolving to more sensitive than their

processors 10 or 30 years ago in world. The sensitivity of the equipment are

increasing and at the same time companies become much stricter to the loss of

production time caused by their decreasing in profit margins besides competitive

ability [3]. Since electronic devices are developed more and more advance

nowadays,

the tolerance and sensitivity to the power quality disturbances for customers have

drastically reduced.

Voltage sag has been considered as one of the most affective power quality

problems for industrial users. It is because voltage sag not only cause technical

problems like power disruptions but also introduces economic loss. Majority of

voltage sags and voltage dips are normally originated by faults [4]. Also, this

problem might cause industrial customers suffer from fluctuation in production rates,

incorrect operation of equipment, flicker of lighting system, tripping of drives, and

inaccurate data obtaining [5].

The most important power quality problem is voltage sags. Based on records

by TNB, 80% of power quality complaints in the world were traced to be related to

voltage sag [6]. Equipment such as process controllers, programmable logic

controllers (PLC) and robotics becoming more sensitive to voltage sags due to the

complexity of the equipment itself increase. This sensitive equipment that used by

industrial customers suffer a huge loss of revenue and it can lead to dangerous

situation because of voltage sag.

4

Mitigation is a very important process. If mitigation could not be done

effectively, it is obviously very hard for the utilities and electrical consumers to

satisfy to the quality of the electricity they obtain. Although there is a lot of power

disturbance improving method being applied to power electrical system other than

DSTATCOM, it provides extremely flexible function in doing this. The reason

behind in this statement is because DSTATCOM are able to improve not only

voltage sag problems, but also the flickers, voltage swell and more power

distribution problems by implementing a proper controller. Also, its control system

act very fast so it could alter the magnitude and phase of voltage almost instantly.

1.3 objectives

The research objectives to be achieved in this project are:

To determine the causes and impacts of power quality problems, specifically

voltage sag.

To simulate the mitigation of voltage sag efficiently using DSTATCOM in

MATLAB

To determine the effectiveness of DSTATCOM in voltage sag mitigation

1.4 Scope

Although there are a lot of power quality disturbances could be observed in

electrical power system, the only power quality to be improved in this project is

voltage sag due to the time constraint. Moreover, the method for voltage sag

mitigation which was discussed in this project is by using Distribution Static

Compensator (DSTATCOM).

5

The modelling and simulation were conducted by using the software of

MATLAB Simulink. The test system to simulate voltage sag problems was designed

and built based on the electrical power system applied in the world. The voltage sag

occurred due to three phase fault, double line to ground fault and single line to

ground fault were simulated. The waveform was analyzed and compensated after

applying DSTATCOM model with special control algorithm to the test system.

1.5 Structure of the thesis

This report consists of five main chapters. In chapter one basically discussed

about related issues with the whole project such as the project background, problem

statement, objectives of the project and the scope of the project. Project background

was discussed about power quality problems nowadays with an increased awareness

of power quality both by customers and utilities. Problems statement and objective of

the project also was mention in this chapter. Scope of the project mention about the

characteristic of the distribution system tested and the type of fault applied in this

project.

For chapter two, it is more details about definition and related theory for

power quality problems and voltage sags. Besides that, a detailed explanation about

D-STATCOM device also include in this chapter.

Next, in chapter three it is discussed about the method used to implement this

project. This project used MATLAB/Simulink software to simulate the distribution

systems condition that is with installed D-STATCOM device and without installed

D-STATCOM device.

Chapter four is about result and discussion. All the results obtained from the

simulation was recorded and discussed in this chapter. Finally, conclusion and

recommendation was made and was stated in chapter five.

6

CHAPTER TWO

LITERATURE REVIEW

2.1 Introduction

The issues like low voltage, voltage dip, voltage sag, etc. have been major

challenge faced by power engineer since three to four decades. Even through

transmit of electricity is getting complex and sophisticated day by day, there is no

end for hunt of a regulated voltage profile, in earlier stages or methods improvement

was done by conventional devices such as synchronous condensers, tap changing

transformer, capacitor placement etc, before the development of a STATCOM ways

to improve power quality is using shunt capacitors to improve power factor, shunt

capacitors are used in rating from 15 KVAR to 10000 KVAR. Small banks of

capacitors up to few hundred KVAR rating are used on individual distribution

circuits of customers. Capacitor banks of 500-3000 KVAR are used in small

distribution substations and those will still larger rating at large substations.

Capacitors are installed either in groups at one central location, say at the primary or

the secondary of transformer or individually on each motor or branch circuit feeding

a group of motors. They are arranged in three phase banks connected in star or delta,

another way to improve power quality is using synchronous condensers to improve

power factor, synchronous condensers is when the KVAR requirements is small, it

can be met through static capacitors. However when requirements exceed 10,000

KVAR it is generally more economical to use the synchronous condensers [7].

7

A synchronous condenser is essentially an over excited synchronous motor;

generally it does not supply any active mechanical power. The excitation of the

machine is varied to provide the necessary amount of the leading KVAR, but a new

dimension has achieved for improvement power quality known as DSTATCOM,

DTATCOM is better for above types of improvement because DSTATCOM has

solved more problems such as low power factor, voltage sag, voltage dip, harmonics

etc, while other two types of improvement only fix poor power factor and etc, that is

the why of DSTATCOM is more important in our distribution system.

2.2 Power quality problems and solutions

2.2.1 Introduction

A power quality problem is defined as "an incidence manifested in voltage,

current or frequency deviations, which results in failure or disoperation of end-use

equipment” Commercial customers have become more exacting in their command

for relative 'quality' of power they purchase, variations in flow or voltage can

actually damage and disrupt sensitive electronics equipment like computers and

microprocessors.

Customers and utilities have a shared responsibility in the mitigation of

voltage variation. Mitigation of the effects on consumer devices from voltage

variations can be achieved only if utilities work with manufacturers in the design of

consumer products so that the products function during normal utility operation.

Different types of power quality problems, customer load profiles and power quality

improvement techniques are discussed in this chapter.

8

2.2.2 Power Quality Problems

Power quality is influenced among other factors by utility operations,

customer load types and equipment designs. Distribution utilities and their

customers, along with their engineering equipment manufacturers and vendors,

generate, propagate and receive power quality problems. Electrical disturbances can

develop from problems within the customer's facility, even though the supply voltage

is constant. Achieving power quality demands a united effort between the utility and

the customer [8].

2.2.2.1 Interruption

An interruption is defined as the complete loss of supply voltage or load

current. Refer to Fig 2.1 for an illustration of an interruption. Within this definition

there are three types of interruptions which are characterized by their duration. The

momentary interruption is defined as the complete loss of supply voltage or load

current having duration between 0.5 cycles and 3 seconds.

Fig: 2.1 Very short interruptions

9

The temporary interruption is the complete loss lasting between 3 seconds

and 1 minute and the long term interruption or outage is an interruption which has

duration of more than 1 minute. The causes of interruptions are myriad and too

numerous to outline in detail but normally result in the operation of a system

protective device, e.g., a fuse or automatic breaker, which is utilized to isolate the

source of the system fault. Common sources of interruptions include lightning,

animals, trees, vehicle accidents and equipment failure.

2.2.2.2 Sags

Voltage sags (dips) are short-duration reductions in rms voltage caused by

short-duration increases of the current, typically at another location than where the

voltage sag is measured. The most common causes of over currents leading to

voltage sags are motor starting, transformer energizing and faults [9].

Fig: 2.2 Long interruptions

10

Voltage sags have been mainly associated with short circuit incidences. Fault

occurrences elsewhere can generate voltage sags affecting consumers differently

according to their location in the electrical system. Starting large motors can also

generate voltage sags, although usually not so severe.

2.2.2.3 Swells

Swell is an RMS increase in the AC Voltage, at the power frequency, for

duration from a half a cycle to a few seconds. Voltage can rise above normal level

for several cycles to seconds.

Voltage swells can originate internally in building wiring or externally on

power lines. Voltage swells are the least frequent of the power line problems

representing only about 2 to 3% of all power problems occurring to industry studies

[9].

Fig: 2.3 Voltage sag or dip

11

Voltage swells will normally cause damage to lighting, motor and electronic

loads and will also cause shutdown to equipment. With electronically controlled

equipment, voltage above 6 to 10% above normal may result in damage.

Solutions to voltage swells for motor loads include motor phase protectors,

electronically controlled devices that shutdown motors before damage occurs. For

sensitive computer and electronic loads, solutions include Uninterruptible Power

Supplies, Voltage Regulators, Power Conditioners, Energy Storage Devices and

Static Switches.

2.2.2.4 Harmonics

When electronic power converters first became commonplace in the late

1970s, many utility engineers became quite concerned about the ability of the power

system to accommodate the harmonic distortion. Many dire predictions were made

about the fate of power systems if these devices were permitted to exist. To some,

harmonic distortion is still the most significant power quality problem. It is not hard

to understand how an engineer faced with a difficult harmonics problem can come to

Fig: 2.4 Voltage swell

12

hold that opinion. Harmonics problems counter many of the conventional rules of

power system design and operation that consider only the fundamental frequency

[10].

2.2.2.5 Harmonic Distortion

Harmonic distortion is caused by nonlinear devices in the power system. A

nonlinear device is one in which the current is not proportional to the applied

voltage. While the applied voltage is perfectly sinusoidal, the resulting current is

distorted. Increasing the voltage by a few percent may cause the current to double

and take on a different wave shape. This is the source of most harmonic distortion in

a power system

Fig: 2.5 Harmonics

13

2.2.2.6 Harmonic Sources from Commercial Loads

Commercial facilities such as office complexes, department stores, hospitals

and Internet data centers are dominated with high efficiency fluorescent lighting with

electronic ballasts, adjustable speed drives for the heating, ventilation and air

conditioning loads, elevator drives and sensitive electronic equipment supplied by

single phase switch-mode power supplies. Commercial loads are characterized by a

large number of small harmonic-producing loads. Depending on the diversity of the

different load types, these small harmonic currents may add in phase or cancel each

other. The voltage distortion levels depend on both the circuit impedances and the

overall harmonic current distortion.

2.2.2.7 Harmonic Sources from Industrial Loads

Modern industrial facilities are characterized by the widespread application of

nonlinear loads. These loads can make up a significant portion of the total facility

loads and inject harmonic currents into the power system, causing harmonic

distortion in the voltage. This harmonic problem is compounded by the fact that

these nonlinear loads have a relatively low power factor.

14

2.2.2.8 Voltage Spikes

Very rapid variation of the voltage value for durations from a some

microseconds to few milliseconds is Voltage spike. These variations may accomplish

thousands of volts. This is due to lightning, switching of lines or power factor

improvement capacitors, disconnection of heavy loads.

2.2.2.9 Voltage Unbalance

Voltage differences in a three phase system in which the three phase angle

differences or the three voltage magnitude differences between them are not equal.

Fig: 2.6 Voltage spikes

15

2.2.3 Source of Power Quality Problems

2.2.3.1 Power Electronic Devices

Power electronic devices viz. Rectifiers, Inverters, Choppers etc. are

nonlinear loads that produce harmonic distortion and can be vulnerable to voltage

sags if not sufficiently protected.

2.2.3.2 It and Office Equipment

IT equipment power provisions consist of a switched mode power supply

(SMPS) and are the reason of a significant increase in the level of 3rd, 5th and 7th

Fig: 2.7 Voltage Unbalacne

16

harmonic voltage distortion in current years. Because the third harmonic is a 'triplen'

harmonic it is of zero order phase sequence and therefore adds in the neutral of a

balanced three-phase system. The increasing use of IT equipment has led to concern

of the increased overloading of neutral conductors and also overheating of

transformers.

2.2.3.3 Arcing Device

Electric arc furnaces, arc welders and electric discharge lamps are all forms

of electric arcing devices. These devices are highly non linear loads. The results of

arc furnaces are difficult to mitigate, balancing the phases with other furnaces will

not always be effective as arc furnaces are operated in various modes, leading to

phase imbalance. Arc welders normally cause transients in the local network due to

the intermittent switching and therefore some electronic equipment may necessitate

safeguard from the impulsive spikes generated [11].

2.2.3.4 Load Switching

The result of load switching on the voltage is normally encountered in the

form of transient activity. This kind of transient might occur as the effect of

switching in a heavy single-phase load [12]. Other apparatus can be protected from

these switching transients by electrically isolating them from the affecting apparatus.

17

2.2.3.5 Large Motor Starting

The energetic nature of induction machines means that they draw current

depending on the mode of operation, during starting this current can be as high as six

times the standard rated current. This increased loading on the local network has the

effect of causing voltage sag, the magnitude of which is reliant on the system

impedance.

2.2.3.6 Sensitive Equipments

Equipment manufacturers are planning and manufacturing ever more

complicated equipment, much of which is increasingly vulnerable to variations in

power quality. There are many concerns relating to the subject of equipment

sensitivity and the effect of power quality occurrences on perceptive equipment.

2.2.3.7 Storm and Environment Related Damage

Lightning strikes are a reason of transient over voltages frequently leading to

faults on the electricity supply network. Lightning does not have to strike a

conductor in order to introduce transients onto the local network. Impulses can be

made if lightning strikes near a conductor. The local ground potential can be move

up by a nearby strike leading to neutral current flowing to earth via a remote ground.

18

2.2.3.8 Capacitor Switching

The capacitor bank is being switched on to compensate for reactive power

losses to maintain the voltage and energy transmission competence of the

transmission grid [13]. Notice that this wave shape also has the wave shape

disturbance occurring at the peak of the sine wave.

2.2.4 Power Quality Improvement Techniques

Nonlinear loads generate harmonic currents that can propagate to other

locations in the power system and ultimately return back to the source. Therefore,

harmonic current propagation creates harmonic voltages throughout the power

systems [14]. Many mitigation techniques have been proposed and implemented to

sustain the harmonic voltages and currents within recommended levels.

High power quality equipment design,

Harmonic cancellation,

Dedicated line or transformer,

Optimal placement and sizing of capacitor banks,

Fig: 2.8 Disturbance caused by the energization of a capacitor bank

19

Derating of power system devices, and

Harmonic filters (passive, active, hybrid) and custom power devices such as

active power line conditioners (APLCs), DVR, DSTATCOM and UPQC.

2.3 Voltage Sag

The definition of voltage sag is a short decrease in voltage magnitude for

duration of time. Based on the IEEE defined standard in the year 1995, the definition

of voltage sag is the reduction of rms voltage from 0.1 to 0.9 pu for a duration of 0.5

cycle to 1 minute [15]. Meanwhile, the meaning of voltage sag might be varied in

terms of duration and magnitude depending on the authority.

It is clarified in terms of duration and retained voltage, normally been

described by using nominal rms voltage remaining at the lowest point when sag

occurs. In other words, voltage sag is also defined as the full energy needed is unable

delivered to the load, which might bring serious effect depends by the load type [16].

2.3.1 Sources of voltage sag

There are three main sources of voltage sags. One of the causes is starting of

large motor loads either on the affected site or by a consumer on the same circuit.

Secondly, it may also originated by the faults on other branches of the supply

network. Thirdly, it might because of the faults in the internal supply scheme of the

consumer’s installation [16].

Voltage sags noticed on the supply network is primarily originated by the

electric short circuit on the electrical supply system. The impact of short circuit is

current rise substantially and hence bigger voltage drops in the impedances of the

20

supply system. The major cause fundamentally contain a breakdown in the dielectric

between 2 structures are kept at different potentials. These structures are proposed to

be insulated from each other [16].

It is a fact that overvoltage is the main culprit contributes to short circuit by

forcing the insulation over its ability. One of the natural phenomena that contribute

to this observation is lightning. Besides, there are some occurrences can weaken,

devastate and bridge the insulation. Such as consequences of other weather effects,

the knocking or contact of animals, vehicles, excavating equipment as well as the

ageing effect [16].

Conveying energy from multiple sources to multiple loads, like motors,

lighting, heating and electronic devices, is conducted by the typical electricity supply

system. The whole system is a single, integrated and dynamic system. It brings the

meaning of any different of voltage, current, impedance, etc. at one point

immediately cause a difference at every other point on the system [16].

Since our country is implementing three phase supply systems, the short

circuit will happen between phases, phases and neutral or phase and ground. When

short circuit occurs at a point, the voltage on the system is dropped to approximately

the same at almost every other point. In order to recover this problem in supply

systems, protective devices are installed to isolate the short circuit from the source.

This sudden declining and companion with recovery stated of voltage are called as

voltage sags [16].

2.3.2 Factors of Voltage Sag

There are various factors can induce the voltage sag problem in real world.

Normally, it is brought by the abrupt increase in loads, especially inductive loads. As

21

we mentioned in previous texts, short circuits or usually known as faults are the

major coming source of the voltage sag occurrence.

Some characteristics of loads like the switching of large loads, starting of

transformers, energizing of large motors and the fluctuation of large magnitude are

all potentially convert the current to effect which is very much similar to short circuit

current. As a result, the resulting changes of voltage will also similar to the

drastically increasing voltage in short circuits. In this situation, they could also be

classified as voltage sags [16].

Due to the supply and installation of cable are dimensioned for normal

running Current, the starting current is very large and affect voltage to fall in the

supply network As well as installation site. This kind of voltage sags caused by

initial currents will occur from one to a few seconds or ten seconds, which is much

longer and less deep to the network fault voltage sags [16].

In short, the causes of voltage sags could be summarize by following [17].

1. Switching of heavy loads

2. Unbalanced load of a three phase system.

3. Remote of rural location from power system.

4. Long distance from a distribution transformer with interposed loads.

5. Grid systems which is not reliable.

6. Equipment not appropriate for local supply.

2.3.3 Types of Faults That Cause Voltage Sag

In this thesis, voltage sags which are caused by faults in an electrical power

system, are the main concerns and were discussed in detail. There are three type of

faults that will probably cause the voltage sags to occur, such as single line to ground

fault, double line to ground fault, and three phase fault [18]. The three phase fault is

22

categorized as balance or symmetrical fault while single line to ground fault and

double line to ground fault are belong to unsymmetrical faults. Three of the faults

will lead to the occurrence of voltage sags with different waveforms.

2.3.3.1 Single Line to Ground Fault

Single line to ground (SLG) fault is the most common fault which will occur

in real electrical power system. The cause of SLG fault might due to lightning

strikes, tree branches, animal contact like bird contact, and more. It is not a fresh

situation that single phase voltage dips to thirty percent of the nominal voltage or less

than that occurred in industrial plants [16].

2.3.3.2 Double Line to Ground Fault

Double line to ground (DLG) fault might happen due to bad weather,

wrecking of the utility poles and tree branches. In this case, two phase voltage sag

will occur and might be noticed on other feeders from same situation [16].

2.3.3.3 Three Phase Fault

Three phase fault is the least to be noticed but it still has the probability to

cause voltage sag and should not be neglected. The situation that may cause three

phase fault to exist is switching or tripping of three phase circuit breaker or switch,

which will introduce the three phase voltage sags in the system. Meanwhile, starting

a large motor will also lead to the happening of three phase voltage sag in a electrical

power system [16].

23

2.3.4 Impact of Voltage Sag

As discussed in Section 2.3, voltage sag will cause abnormalities in voltage

magnitude reduction. These impacts might affect users and utility, and it is a very

annoying problem to the industrial customers. Hence, it is quite crucial for modern

engineers to deeply understand the main effect and obvious impact so that the power

quality could be improved to maximum level. It is not only beneficial to all the

electrical equipment users but also contribute to the enhancement of electrical field

reputation. Perhaps the customers will also lost confident and make complains to the

electricity supplier once the reputation degrade.

In general, the effects of voltage sags mainly contributed to production rates

fluctuates, abnormal functioning equipment, dimming of lighting systems, variable

speed drives shut down to avoid damage, relays and contractors drop out and even

the unreliable data in equipment test or calibration [18].

The symptoms of voltage sags might not obvious and common for residential

customers but it is not a fresh issue for industrial users. Therefore, it is encouraging

to study the effects of voltage sag to industrial equipment. Electrical equipment

which are now so integral to industrial and commercial power system, will easily

break down if exposed to a voltage, current or frequency deviation. Before solid-state

electronics are well developed, power quality was not studied since the impact on

most loads connected to electrical distribution systems is ignorable. When an

induction motor underwent voltage sag, it did not turn off automatic but only operate

in less power until the sag was over. The same situation happened on incandescent or

fluorescent lighting systems in a facility and it reacted by shortly reduced the lumen

output.

In contrast come to nowadays, contractors and engineers always emphasize

on installing specialized equipment to prevent the happening of unwanted occurrence

due to the high sensitivity equipment and complicated process as well as the

expensive repairing cost.

24

In term of electronic equipment, the effect of voltage sags is meaning about

how much energy is being transferred into the power supply. If insufficient energy is

passing into the power supply which is caused by voltage sag, the DC voltage

received by the IC decrease resulting to the shutting down, locking down or data

garbling of device to occur. If the device shuts down, it will normally restart once the

energy return into the supply. It is because electronic devices need a more

manipulated electrical environment than most other loads. For instance,

programmable logic controls (PLC) system and inverter.

On the other hand, motors are vastly tolerant to voltage sags besides voltage

Swells. Motors only have very small response to voltage variations except during the

experiencing of extremely low or extremely high r.m.s magnitudes. Since motors are

manipulated by electronic drive controllers, the effect on electronic equipment will

also be concerned. When the magnitudes are abruptly large or voltage sags occur

often, it will stress the windings on the stator, which is potentially damage the

premature motor. Besides that, extreme sags may also leads to insufficient rotational

inertia and resulting to decline of performance or effectiveness. Also, if it occur

frequently, the motor may draw high inrush currents often enough to trip a breaker.

Furthermore, majority of the lighting system are tolerant of voltage sags and

Voltage swells. Incandescent system will simply or continuously light brighter or

dimmer and make it produce annoying visual effect besides overall lifespan might be

affected. The sudden change in brightness is usually known as flicker.

For sensitive equipment in industrial plants, it is a fact that severe swells may

stress components to the point of broken down, but less disturbance or damage could

be notice other than that. The responsiveness of system towards load when expose to

voltage sag might determine the problems impact. It is hard to deny that huge value

of voltage sag could result in tripping of breakers, blowing of fuses or damage the

electronic components such as PLC control devices.

25

2.4 Custom power devices

2.4.1 Dynamic Voltage Restorer

2.4.1.1 Introduction

Dynamic Voltage Restoration (DVR) is a method and apparatus used to keep

up, or restore, voltage sags, or spikes in voltage supply to sustain operational electric

load. frequently used in manufacturing areas requiring significant power to run

tools/equipment, and utility plants, this custom device mitigates potential damage to

equipment and undesirable slowdowns to the production line caused by an abrupt

change in electric load. This method uses critical devices such as an automatic

transfer switch and IGBT Modules in order to operate. DVRs are now a developed

option in industry to decrease the impact of voltage sags to sensitive loads. The use

of DVR in power quality applications is rising. Although, the most popular

application of DVR is to manage voltage sags (swells) but the harmonics and power

factor correction may also be achieved through robust control schemes. To get a

number of benefits to industrial, commercial, and residential customers is by

employing and installing DVR in whole system. Reduce in shut down time of

process industries, small losses in the production process and reduction of insulation

damage on transformers, and smooth operation of sophisticated electronic

equipment’s are few of them. Proliferation of sensitive load has opened the venues

for DVR application in all consumer categories. DVR has variety applications in

transmission and distribution systems. It is a series compensation device, DVR has

more protection to sensitive electric loads from power quality problems such as

voltage sags, swells, unbalance and distortion through power electronic controllers

that use voltage source converters (VSC). The first DVR was installed in North

America in 1996 - a 12.47 kV system located in Anderson, South Carolina. Since

then, DVRs have been applied to defend critical loads in utilities, semiconductor and

26

food processing. Today, the most efficient power quality devices in solving voltage

sag harms are the dynamic voltage restorer. The basic theory of the dynamic voltage

restorer is to inject a voltage of required magnitude and frequency, so that it can

restore the load side voltage to the desired amplitude and waveform even when the

source voltage is unbalanced or distorted. usually, it employs a gate turn off thyristor

(GTO) solid state power electronic switches in a pulse width modulated (PWM)

inverter structure. The DVR can generate or absorb independently controllable real

and reactive power at the load side. In other words, the DVR is made of a solid state

DC to AC switching power converter that injects a set of three phase AC output

voltages in series and synchronism with the distribution and transmission line

voltages [19]. The schematic diagram of dynamic voltage restorer (DVR) is shown in

figure 2.9

2.4.1.2 DVR Components

Voltage Source Converter (VSC): a voltage source converter is a power

electronic system consisting of a storage device and switching devices, which can

generate a sinusoidal voltage at any required frequency, magnitude, and phase angle.

When VSC is in the DVR can temporarily restore the supply voltage or to generate

Fig: 2.9 Dynamic Voltage Restorer(DVR) schematic diagram

27

the part of the supply voltage which is missing. The missing voltage is the difference

between the nominal voltage and the actual. The VSC is based on some kind of

energy storage that will provide the converter with a DC voltage. The solid state

electronics in the converter is then switched to get the desired output voltage [20].

Injection transformer: injection transformers employed in the dynamic

voltage restorer plays a vital role in ensuring the maximum reliability and

effectiveness of the restoration scheme. It is connected in series with the distribution

feeder. he Injection / Booster transformer is a specially designed transformer that

attempts to limit the coupling of noise and transient energy from the primary side to

the secondary side. Its main tasks are first It connects the DVR to the distribution

network via the HV-windings and Transforms and couples the injected compensating

voltages generated by the voltage source converters to the incoming supply voltage.

Second In addition, the Injection transformer serves the purpose of isolating the Load

from the system (VSC and control mechanism [20].

Passive Filters: passive filters are used to convert the PWM inverted pulse

waveform into a sinusoidal waveform. This is achieved by removing the unnecessary

higher order harmonic components generated from the DC to AC conversion in the

VSC, which will distort the compensated output voltage. These filters can be placed

either in the high voltage side (load side) as shown in figure 2.10 or in the low

voltage side (inverter side) of the injection transformers as shown if figure 2.11 . The

advantage of the inverter-side filter is that it is on the low-voltage side of the series

transformer and is closer to the harmonic source. Using this scheme, the high-order

harmonic currents will be prevented from penetrating into the series transformer thus

reducing the voltage stress on the transformer [20].

28

Energy storage device: During voltage sag, the DVR injects a voltage to

restore the load supply voltages. The DVR needs a source for this energy. That

source of energy is an energy storage device. Some energy storage devices are dc

capacitors, batteries, super-capacitors, superconducting magnetic energy Storage and

flywheels. The capacity of energy storage device has a big impact on the

compensation capability of the system. Compensation of real power is essential when

large voltage sag occurs [20].

Fig: 2.10 Filter placed in high voltage side

Fig: 2.11 Filter placed in Low voltage side [20]

29

2.4.1.3 Operating Principles of DVR

In figure 2.2 is a DVR compensated single phase system. Let us assume that

source voltage is 1.0 Pu and we want to regulate the load voltage to 1.0 Pu. Let us

denote the phase angle between V s and VL as δ. In this analysis, harmonics are not

considered. Further we assume that during DVR operation, real power is not required

except some losses in the inverter and the non ideal filter components. These losses

for the time being are considered to be zero. This condition implies that the phase

difference between Vf and is should be 90o. Let us first consider a general case to

understand the concept [21].

Apply Kirchhoff voltage law (KVL) in circuit of figure 2.12 is

Fig: 2.12 Schematic diagram of a DVR based compensation in adistribution system

30

Note that in the above circuit Is = IL =I. The load voltage VL can be written

in terms of load current and load impedance as given below

Using (2.1), the source voltage can be expressed as in the following.

With the help of above equation, the relationship between load voltages and

source voltage and DVR voltages can be expressed as below

2.4.2 Unified Power Quality Conditioner (UPQC)

2.4.2.1 Introduction

UPQC is a custom power device and consists of combined series active

power filter and shunt active power filter. Series active power filter that compensates

2.1

2.2

2.3

2.4

31

voltage harmonics, voltage unbalance, voltage flicker, voltage sag/swell and shunt

active power filter that compensates current harmonics, current unbalance and

reactive current. UPQC is also known as universal power quality conditioning

system, the universal active power line conditioner and universal active filter. It is a

common operation of series and shunt active conditioner. Shunt active power filter

have capability of the current compensation, series active power filter have capability

of voltage compensation allow mitigation of various power quality problem. The

function of unified power quality conditioner is to eliminate the disturbances that

affect the performance of the critical load in power system. In other words, the

UPQC has the capability of improving power quality at the point of installation on

power distribution systems. The UPQC, therefore, is expected to be one of the most

powerful solutions to large capacity loads sensitive to supply voltage flicker and

voltage unbalance. The UPQC, which has two inverters that share one dc link, can

compensate the voltage sag and swell, the harmonic current and voltage, and control

the power flow and voltage stability. Besides, the UPQC can also compensate the

voltage interruption if it has some energy storage or battery in the dc link [22].

2.4.2.2 Classification of UPQC And Its Operating Principle

The UPQC are classified in many different ways based on: a power inverter

topology that is used as the power conversion unit. The system configuration of the

conventional UPQC consists of the shunt active power filter that is placed on the

right side with respect to the series active power. Both the shunt and series active

power filters are based on the six-switch VSC topology. Both the shunt and series

VSCs are connected to a common dc-link. In the three phase three wire DG system,

only the three-phase balanced sensitive and non-linear load can be connected due to

inexistence of a neutral wire. Thus, the three-phase source currents which flow

through the distribution network are balanced and (3n + 3) harmonic free, where n =

0, 1, ∞. A single line representation of UPQC as shown in figure 2.13 [22].

32

The shunt active power filter is connected in parallel with respect to the

distribution network. It is controlled in the current control mode and solves current

power quality problems, thus, it operates as a controlled current source. Another key

component of the shunt active power filter is its interfacing passive filter. Its

configuration typically varies among two types such as a coupling inductor and a

parallel LC filter [23]. The main function of all of these types is to mitigate high-

frequency current switching harmonics. Moreover, the choice of the passive filter

type generally depends on a selected control method for the shunt active power filter.

The coupling inductor is typically used with linear controllers. Non-linear control

techniques might be implemented in combination with the parallel LC filter [24].

The series active power filter is connected in series with the distribution

network using three single phase transformers. It operates as a controlled voltage

source and handles voltage related power quality problems we mentioned in above

section. In contrast to the shunt active power filter only one type as the parallel LC

filter is employed in the series active power filter [25]. Similarly, it compensates

high-frequency voltage switching harmonics. The dc-link in the conventional UPQC

Fig: 2.13 Single line representation of conventional UPQC

33

is usually formed as a single capacitor. It interconnects two VSCs, which create well

known back-to-back topology, and maintains constant dc-bus voltage, if the power

storage unit is not connected. Thus, different dc-bus voltage control techniques are

developed such as: the proportional-integral (PI) - controller-based approach, fuzzy-

PI controller, optimized controller, PID controller and unified dc voltage

compensator to name a few. The PI-controller-based approach is the most commonly

used.

2.4.3 DSTATCOM

2.4.3.1 Introduction

STATCOM is stand for Static Compensator. It is one of the FACTS family

devices. As we know FACTS devices stand for Flexible AC Transmission Systems.

It consists of a group of power electronic devices such as IGBT, GTO and transistor.

FACTS Devices functioning as same as other power system controllers such as

transformer tap changers, phase shifting transformers, passive reactive compensators,

and synchronous condensers [26]. To categorize the FACTS Devices, it can be seen

by the way they connected to the power systems, either in shunt, series on in shunt-

series connection.

STATCOM can be used on alternating current electricity transmission

networks. Basically a STATCOM is a system that relates closely with power

electronic device. One of the power electronic device that be used in STATCOM is

voltage source converter (VSC) .Voltage source converter functioning as a source or

supplier. It will provide a reactive AC and active AC power to an electrical system.

Usually a STATCOM is installed to support electricity networks that have a poor

power factor and often poor voltage regulation. Besides that, STATCOM can also be

used in wind energy, voltage stabilization, and harmonic filtering. It also may be

34

used for the dynamic compensation of power transmission system, providing voltage

support and increased transient stability margins. However, the most common use of

STATCOM is for voltage stability [27].

The concept of STATCOM was disclosed by Gyugyi in 1976, In 1995, the

first 100 MVA STATCOM was installed at the Sullivan substation of Tennessee

Valley Authority in northeastern Tennessee, United States. This unit is mainly used

to regulate 161 kV bus during the daily load cycle to reduce the operation of the tap

changer In 1996, the National Grid Company of England and Wales designed

dynamic reactive compensation equipment with inclusion of a STATCOM of 150

MVA range. Confidence in the STATCOM principle has now grown sufficiently for

some utilities to consider them for normal commercial service. Japan (Nagoya),

United States, England and Australia (QLD) also use STATCOM in practice.

The general arrangement of STATCOM is shown in figure 2.14 .STATCOM

system functioning as same as static VAR Compensator (SVC). Both of them

provide shunt compensation by using a voltage source converter. The basic principle

of operation of STATCOM is generation of a controllable AC voltage source behind

a transformer leakage reactance by a voltage source converter connected to a DC

capacitor. The voltage difference across the reactance produce active and reactive

power exchanges between the STATCOM and power system [26].

A D-STATCOM consists of Voltage Source Converter (VSC), a DC energy

storage device, a capacitor, and a coupling transformer to connect the D-STATCOM

through it in shunt to the distribution network.

35

2.4.3.2 Main Components of (D STATCOM)

D-STATCOM consists of three main components that is Voltage Source

Converter (VSC), Energy Storage Circuit, and it Controller system. Each one of this

component play an important role to ensure that D-STATCOM can operate wisely

without have any problems.

Voltage Source Converter (VSC) is one of the power electronic devices.

VSC is the most important component in D-STATCOM and it can generate a

sinusoidal voltage waveform with any required magnitude, with any required phase

angle and also with any required frequency. Usually VSC is mostly used in

Adjustable Speed Drive but it also can be used to mitigate the voltage sags. VSC is

used to replace the voltage or to inject the ‘missing voltage’. The missing voltage can

be defined as the difference between the actual voltage and the nominal voltage [29].

Fig: 2.14 General arrangement of statcom

36

Normally, the converter is based on some kind of energy storage which will get the

supply from the DC voltage. This converter is used the switching based on a

sinusoidal PWM method. The PWM offers simplicity and good response. The device

that used for the switching is an IGBT power electronic device.

Energy Strorage Circuit: The purpose of energy storage is to maintain the

DC side voltage of VSC. It can be capacitor or DC source, e.g. battery. Traditional

STATCOM only has DC capacitor, thus; only reactive power can be injected to the

power system by STATCOM, whereas both active and reactive power can be

injected to the power system by STATCOM if DC source is used. In energy storage

circuit, the DC source was connected in parallel with the DC capacitor. DC source is

act as a battery that will supply a power meanwhile the DC capacitor is the main

reactive energy storage element. It carries the input ripple current of the converter.

To charged the DC capacitor, it could be used either a battery source or it could be

recharged by the converter itself [30].

Filter and Control part: As the Pulse-Width Modulation (PWM)

technique is used in VSC, the output voltage of VSC has switching ripple, which

bring harmonics into the current injected to the power system. These harmonics will

affect the voltage quality of the power system. Therefore, a relatively small reactor is

installed between VSC and the point of the system which the D-STATCOM is

connected, to filter those harmonics in the current. The filter can be L-filter, LC-filter

and LCL-filter.

The aim of the controller system is to maintain the constant voltage

magnitude at the point where a sensitive load is connected under system

disturbances. The control system element can only measured the RMS voltage

magnitude that measured at the load point. For the controller system there is no

requirements of the reactive power measurements. The input for the controller

system is an error signal. This error signal is obtained from the reference signal

measured at the terminal voltage and RMS voltage magnitude that measured at the

load point. First of all, this error signal will enter to the sequence analyzer block

37

which is functioning to measure the harmonic level in that signal. Then, the PI

controller will process this error signal and come out with the output in term of the

angle, ∂. This angle can drive the error to zero. Next, this angle will be summed with

the phase angle of the supply voltage which is assumed to be 120 to produce the

suitable synchronizing signal, required to operate the PWM generator [31]. Then,

this angle will be submitted to the PWM signal generator. PWM generator will

generate the sinusoidal PWM waveform or signal.

2.4.3.3 Distribution Static Compensator Configuration

In its most basic form, the STATCOM configuration consist of a two level

voltage source converter (VSC), a dc energy storage device, a coupling transformer

connected in shunt with ac system, and associated control circuit [28].Figure 2.15

depicts the schematic diagram of the STATCOM. The VSC will convert the DC

voltage across the storage device into AC output voltages that are in phase and

coupled with the AC system through the reactance of coupling transformer. Since the

AC output voltage connected directly with the coupling transformer, the exchange of

active and reactive powers can be easily made between the converters and the AC

system. The active and reactive power can be exchanged directly by adjusting the

phase angle between the converter output voltage and the bus voltage at the point of

common coupling.

38

There are many types of FACTS device such as Static Var Compensator

(SVC), Dynamic Voltage Restorer (DVR) and STATCOM itself. To see the

advantages of STATCOM, it can be compared with the Static Var Compensator

(SVC). There is a few main advantages of STATCOM over the conventional Static

VAR Compensator (SVC) [32]. Firstly, STATCOM has significant size reduction

due to reduced number of passive elements. Then, it is also can be able to supply

required reactive power even at low voltages. Next, STATCOM is a creator reactive

power current output capability at depressed voltages and it is also exhibits faster

response and better control stability.

2.4.3.4 DSTATCOM V-I Characteristic

The STATCOM can be operated in two different modes which is in voltage

regulation mode and can operate in reactive power control mode. When the

Fig: 2.15 The schematic diagram of a STATCOM

39

STATCOM operated in reactive power mode, the STATCOM reactive power output

itself will remain constant. When the STATCOM is operated in voltage regulation

mode, it will produces the following Voltage versus Current (V-I) characteristic.

Figure 2.16 show the V-I characteristic of STATCOM. The STATCOM can supply

both the capacitive and the inductive compensation and is able to independently

control its output current over the rated maximum capacitive or inductive range

irrespective of the amount of ac-system voltage. The STATCOM can provide full

capacitive-reactive power at any system voltage. This capability is useful for

situations in which the STATCOM is needed to support the system voltage during

and after faults where voltage collapse would otherwise be a limiting factor.

From the figure 2.16 it show that as long as the reactive current stays within

the minimum and minimum current values (-Imax, Imax) imposed by the converter

rating, the voltage is regulated at the reference voltage Vref. However, a voltage

droop is normally used (usually between 1% and 4% at maximum reactive power

output), and the V-I characteristic has the slope indicated in the figure. In the voltage

regulation mode, the V-I characteristic is described by the following equation:

Slope xs Vref

Fig: 2.16 DSTATCOM V-I Characteristic

40

Where,

V - Positive sequence voltage (pu)

I - Reactive current (pu/Pnom) (I > 0 indicates an inductive current)

Xs - Slope or droop reactance (pu/Pnom)

Pnom- Three-phase nominal power of the converter

2.4.3.5 Basic Configuration and Function of D-STATCOM

DSTATCOM consist of three main part, namely injection transformer,

voltage source inverter (VSI) and PWM generator with specific control scheme. The

function of injection transformer is to inject the AC produced from VSI. On the other

hand, the VSI is used to convert DC storage to AC while PWM generator is to

generate the appropriate gate signal for the switching device in VSI to perform

voltage sag mitigation function.

The voltage source inverter (VSI) could convert DC voltage into AC

sinusoidal voltage before injection of current back to the power system is done via

injection transformer. The total replacement of voltage or insertion of voltage to fill

the dipped voltage could be done by implementation of this voltage source converter

or specifically named as inverter. A DC energy storage is used to supply the

converter with DC voltage, while the electronic switching devices invert DC into the

output voltage required.

The most important part of DSTATCOM is its controller. By applying

appropriate controller, various power quality disturbances could be solved

41

specifically, includes voltage sag. The main purpose of the control scheme is to keep

voltage magnitude fixed at the point where the power system is undergoing voltage

sag problem. In modern controller, not only controller for voltage sag compensation,

but also some other low-power application use PWM technique instead of

Fundamental Frequency Switching (FFS) methods because PWM is more flexible,

simple, and good response. Also, PWM techniques could be applied with high

switching frequencies so that its efficiency could be maximized and also the

switching losses could be drastically reduced.

The controller of the DSTATCOM is designed to conduct the reactive power

exchange between the inverter and the system line by modifying the phase angle

between the inverter voltage and line voltage. The reactive power output of the

DSTATCOM could either be inductive or reactive, depending on the operation mode

of DSTATCOM. There are three operation modes. When the inverter voltage is same

as the system voltage, no reactive power exchange is conducted. When the inverter

voltage is larger than the system voltage, the DSTATCOM is having the

inductive reactance, the current will be injected from the inverter to the system

through the

Injection transformer. Consequently, capacitive reactive power is generated

by the DSTATCOM . When the inverter voltage is smaller than the system voltage,

the DSTATCOM is responding as capacitive reactance, the current will passes

through the injection transformer from the system to the inverter. Consequently, the

inductive reactive power is absorbed by the DSTATCOM.

42

2.4.3.6 Calculation of Voltage Injection By D-STATCOM

Based on Figure 2.17, the voltage sag is corrected by the shunt injected

current ISH by regulating the voltage dip across the system impedance ZTH. By

modifying the output voltage of the inverter, the value of ISH can be controlled.

Based on Figure 2.11, by using Kirchhoff’s circuit law (KCL):

𝐼𝑠ℎ = 𝐼𝑙 − 𝐼𝑠

𝐼𝑠ℎ =𝑉𝑡ℎ−𝑉𝑙

𝑍𝑡ℎ

So by substituting equation 2.5 into 2.6

𝐼𝑠ℎ = 𝐼𝑙 −𝑉𝑡ℎ−𝑉𝑙

𝑍𝑡ℎ

When angles are considered

Fig: 2.17 Schematic diagram of D-STATCOM

2.5

2.6

2.7

43

𝐼𝑠ℎ∠𝜂 = 𝐼𝑙∠(−𝛼) −𝑉𝑡ℎ∠(𝛼−𝛽)−𝑉𝑙∠−𝛽

𝑍𝑡ℎ

Where𝜂 , 𝛼 and 𝛽 are the angles of shunt inverter voltage, line angle, thevenin

voltage and thevenin impedance respectively.

The complex power exchange at DSTATCOM side is able to be indicated as:

𝑆𝑠ℎ = 𝑉𝑙𝐼𝑠ℎ

Hence, the effectiveness of DSTATCOM in compensating the voltage sag is actually

based on the value of ZTH or fault level of the load bus.

2.2.3.7 Operation Modes of A DSTATCOM

No load mode

If Vi is equal to Vs, the reactive power is zero and the D-STATCOM does

not generate or absorb reactive power.

Fig: 2.18 No load mode( Vs-Vi )

2.8

2.9

44

Capacitive mode

When Vi is greater than Vs, the D-STATCOM shows an inductive reactance

connected at its terminal. The current I, flows through the transformer reactance from

the DSTATCOM to the ac system, and the device generates capacitive reactive

power.

Fig: 2.19 Capacitve mode ( Vi>Vs )

45

Inductive mode

If Vs is greater than Vi, the D-STATCOM shows the system as a capacitive

reactance. Then the current flows from the ac system to the D-STATCOM, resulting

in the device absorbing inductive reactive power.

Fig: 2.19 Inductive mode ( Vs>Vi )

46

CHAPTER THREE

METHODOLOGY

3.1 Introduction

In this chapter will discuss the methodology of this project. And also discuss

about design of 11kv distribution system in order to finish up this project

successfully, the project consists of three parts including designing phase, simulating

and analysis respectively. First phase the designed distribution system with a creation

of voltage sag by three faults is subjected to the effect of distribution static

compensator (D-STATCOM) in enhancing the voltage sag in the distribution

network. Second phase is the modeling of the D-STATCOM had been developed

and simulates using MATLAB/Simulink. Third phase The simulation implemented

on distribution network is without installation of D-STATCOM and with installation

of D-STATCOM to the network system was analyzed.

3.2 Project Flow

In order to accomplish to goal of the project, the following process as shown

in fig 3.1 must be followed carefully.

47

Fig: 3.1 Flow chart of the project process

48

Firstly, for the first phase of the project, the understanding about voltage sag

and methods to improve the problems is compulsory. Every idea and topic related to

the problems was clarified and comprehends it clearly. After that, after the method of

the project was determined, the introduction result should be done. This is the

important part because it is an initial step in order to deal with the bigger scope.

For the second phase of the project, a clear understanding should be made.

Anything related to Distribution Static Compensator (D-STATCOM) scheme should

be done. After that, the simulation was implemented. The result found from the

simulation was compared with theoretical and validate it by calculation if needed. If

the result found from the simulation does not match the theoretical the simulation

needs to run back. Eventually, if the simulation was successful at that time the

analysis was started and after that the report writing should be done.

3.3 simulations

According to the scope of this project, the simulation was tested on the

distribution system. In order to make voltage sags, three phase fault was applied on

feeder B and immediately voltage sag will take place in feeder A. After that,

Distribution Static Compensator (D-STATCOM) will be inserted into the distribution

system to mitigate the voltage dips or sags.

49

Fig: 3.2 Flow chart of simulation process

50

Figure 3.2 shows the simulation process flow. That flow chart clarifies about

the in general process of the simulation. Initially, the most important circuit of

distribution system was applied in MATLAB/Simulink. Next, to generate voltage

sags, three phase fault with different resistance values was applied on the system.

Next, start the simulation between 0 up to 1 second. at that time, to advance the

voltage sag, DSTATCOM was inserted into the distribution system. If the sags is still

less than 0.9 p.u after the addition of D-STATCOM, run another time the simulation

until the result give details that the sags get better greater than 0.9 p.u. Then the

outcome from the scope can be studied.

3.3.1 System implementation in MATLAB/Simulink

Based on the scope of this project, the simulation was experienced on the

distribution system. The experiment system consists of a 275 kV transmission

system, feeding into the primary side of a 3-winding transformer. A differentiating

load is connected to the 11 kV, secondary side of the transformer. In order to

examine the sags (dips) event, three lines to the ground fault was created. Then,

automatically voltage sags (dips) will happen. therefore, we can analyze the result of

using Distribution Static Compensator (D-STATCOM) and how D-STATCOM

systems enhance the sags (dips). All of this process was completed by using

MATLAB/Simulink.

51

Initially, the major circuits was designed and fulfilled in the

MATLAB/Simulink. Figure 3.3 illustrates the main circuit of distribution network

without setting up of D-STATCOM systems. All these values of the every part is

shown in Appendix A. the linked load to the system is balanced loads and thus load

is connected to the 11 kV, secondary side of the transformer. The three phase fault

creator was located on feeder B. The run time for the entire system is 1 second where

sags (dips) happen in feeder A for the period of 0.3 second to 0.6 second.

Fig: 3.3 The main circuit implement in MATLAB/simulink

Fig: 3.5 Sample run time for the simulation

52

Figure 3.4 displays that the fault maker was placed to the three phase to the

ground and the transition times were put to 0.3 to 0.6 in order to create sag for 0.3

seconds. Then three phase voltage measurement located at feeder A. The outcome of

this component will be sent to the scope in order to examine the sags (dips) at feeder

A. also that, RMS signal is besides located at the scope in order to calculate the sags

(dips) at the load end. Then the simulation was established by clicking the run button

close to simulation displayed run time as shown in Figure 3.5. The effect of the

simulation without installation of (D-STATCOM) can be seen from Scope 1 by

Fig: 3.4 Configuration of fault generator

53

double click on it and the result of the voltage sags compute from the load end can be

analyzed from scope 3. Then the result was recorded. After that, the second

simulation is to simulate the power distribution system set up with D-STATCOM

system. While the scope of this project was treat with D-STATCOM and only the

STATCOM scheme present in MATLAB/Simulink, so D-STATCOM system

required to be model. STATCOM and D-STATCOM is not the same, it was

dissimilarity as discuss in chapter two previously. D-STATCOM consists of three

major part also has discuss in chapter two which is Voltage Source Converter

(VSC), Controller system and Energy storage circuit. Each division of this

DSTATCOM require be model in MATLAB/Simulink to make sure a good

performance of the D-STATCOM.

Figure 3.6 explain the Energy storage circuit and Voltage Source Converter

(VSC) correspondingly. DC source is linked in parallel with the DC capacitor. it

takes the input wave current of the converter and it is the major reactive energy

storage component. This DC capacitor could be charged by a battery source or could

be recharged by the converter itself.

A voltage-source changer is a power electronic tool that coupled in shunt or

parallel to the network. Figure 3.7 demonstrate the configuration of this VSC. Power

Fig: 3.6 Energy storage circuit and voltage source converter

54

electronic device that used in this simulation are IGBT with the number of bridge

arm are three.

Fig: 3.7 Configuration of voltage source converter

Fig: 3.8 Control scheme design for the D-STATCOM

55

Referring to figure 3.8 it displays the control system for the D-STATCOM.

The controller scheme is partially part of distribution system. Proportional-integral

controller (PI Controller) is a feedback controller. This PI controller will guide the

scheme to be controlled with a weighted sum of the error signal and integral of that

value. Error signal can be defined as dissimilarities between the output and desired

set end value. PWM producer is used to make the Sinusoidal PWM waveform or

signal.

Figure 3.9 shows the control mechanism. Control mechanism is one

component of the controller system. It is a phase modulation control angle.

To make sure that the usefulness of this controller in gives that continuous

voltage regulation, simulations were approved with and without D-STATCOM

associated to the network. A two level DSTATCOM is linked to the 11 kV tertiary

winding to give immediate voltage support at the load end. Voltage sags can take

Fig: 3.9 Control Mechanisms

56

place when there is fault on the distribution line and to enhance the voltage sag,

FACTS device similar to DSTATCOM has been generate to solve this difficulty.

Fig: 3.10 D-STATCOM system connected to the distribution system

57

3.3 Conclusion

In this chapter, it is talked about the way used to implement this project. In

this chapter it will clarify more detailed about to generate a voltage sags problem and

to solve it by using D-STATCOM mechanism. MATLAB/Simulink software has

been used and 11kV distribution scheme has been implementing in

MATLAB/Simulink software. first of all, the distribution system with two feeders

has been design. Then to make voltage sags, three phase fault has been applied at one

of the feeder. After that, run the simulation between 0 until 1 second to see the

voltage sags waveform. Then, to advance the voltage sags, D-STATCOM device will

be put in into the distribution system. D-STATCOM consists of controller structure,

Voltage Source Converter (VSC) and energy storage circuit. Afterward, the wave

shape from the scope can be investigated to observe the either the voltage sags had

get better or not with addition of DSTATCOM.

58

CHAPTER FOUR

RESULT AND DISCUSSION

4.1 Introduction

This chapter will focus on the results gained in simulation. The result consists

of the simulation of the distribution system without insertion of Distribution Static

Compensator (D-STATCOM) scheme and with installation of DSTATCOM system.

4.2 simulations without insertion D-STATCOM scheme

Distribution system without D-STATCOM system was implemented in

matlab simulink as shown in Figure 4.1. Reference signal found before three phase

fault was applied in the distribution system as shown in Figure 4.2. Reference signal

for every phase before three phase fault was applied the signal find was normal with

no any voltage drop happen as shown in Figure 4.3.

59

Fig: 4.1 main circuit of Distribution system without D-STATCOM

Fig: 4.1 Main circuit of the distribution system without DSTATCOM

Fig: 4.2 Reference signal before three faults applied

Main circuit of the distribution system without

DSTATCOM

60

In order to make the voltage sags in distribution system, various types of fault

such as and Single Line to Ground (SGL), Three Line to Ground (TLG) And Double

Line to Ground (DLG), are inserted on feeder B. That faults will bring an

interruption on feeder B. on the other hand, feeder A will bring voltage sag instead

the fault actually on the system. It is same goes with the distribution system that has

three feeders or more than that. Once a fault happens on one feeder, the other whole

parallel feeder will bring voltage sag. Figure 4.4 shows the result of the simulation

when three phases to ground fault applied and Figure 4.5 shows the results of voltage

sag for each phase. The result obtained was as expected. Voltage sags occurred

between 0.4s until 0.6s.

Fig: 4.3 Reference signal for each phase A, phase B and phase C before fault respectively

Main circuit of the distribution system without DSTATCOM

61

Fig: 4.4 Reference Signal after Three Phase Fault Applied or voltage sag

Fig: 4.5 Voltage output at feeder A for each phase. Phase A , phase B and Phase C

62

Table 4.1 shows that the results of the voltage sag for different type of fault.

Three different types of fault was applied in the distribution system and the value of

the fault resistance was varies.

Voltage sags for

STLG(p.u)

Voltage sags for

DLG(p.u)

Voltage sags for

TLG(p.u)

Fault resistance

Rf(Ω)

0.4676 0.4671 0.4672 0.66

0.5027 0.5028 0.5024 0.76

0.5314 0.5315 0.5311 0.86

0.5554 0.5550 0.5546 0.96

From the table, it can observe that the value of fault resistance is directly

proportional to the voltage. When the values of fault resistance increase, it shows that

the voltage will also increase for different types of fault. Figure 4.6 (a) until figure

4.6 (c) show the result of voltage sags for different types of fault measure at the load

point when the value of fault resistance is R = 0.66Ω. The faults occur between 0.4s

until 0.6s.

Table 4.1: Results of voltage sags for different type of faults

63

4.3 Simulation with installation D-STATCOM system

Figure 4.7 shows the order of the distribution system with D-STATCOM

system implemented in MATLABsimulink. Figure 4.8 shows the result of

compensation D-STATCOM to the power system. It shows that the custom power

device successfully compensate the voltage sag. Almost 90% of the voltage sag

during 0.4s to 0.6s back to the nominal value. However, since custom power device

Figure 4.6 (a): Voltage at load point is 0.4672 Pu for TLG fault

Figure 4.6 (b): Voltage at load point is 0.5394 Pu for DLG fault

Figure 4.6 (c): Voltage at load point is 0.5149 Pu for TLG fault

64

deal with electronic element, harmonic and distortion will appeared in the system.

Figure 4.9 shows the voltage sag has improved for each phase.

Fig: 4.7 Main circuit of the distribution system with Dstatcom system

Fig: 4.8 Three phase output voltage after compensation

65

Table 4.2 shows that the results of the voltage sag after compensation for

different types of fault. Three different types of fault was applied in the distribution

system and the value of the fault resistance was varies. From the table 4.2, it can be

observed that voltage sags improved with the insertion of D-STATCOM. The value

of voltage sag is between 0.9407 pu until 0.9974 pu.

Figure 4.9: Output voltage at feeder A after compensation for each phase. Phase A, Phase B

and Phase C respectively.

66

Voltage sags for

SLG(p.u)

Voltage sags for

DTLG(p.u)

Voltage sags for

TLG(p.u)

Resistance

Rf(Ω)

0.9858 0.9813 0.9407 0.66

0.9907 0.9878 0.9531 0.76

0.9940 0.9920 0.9615 0.86

0.9974 0.9959 0.9686 0.96

Figure 4.10 (a) until Figure 4.0 (c) show the results of voltage sags after

compensation for different types of fault measure at the load point when the value of

fault resistance is R = 0.66Ω. The fault occurs between 0.4s until 0.6s.

Table 4.2: Results of voltage sags after compensation for different type of fault

Fig: 4.10 (A) Voltage at load point 0.9402 Pu for TLG fault

67

It can be observed that, after compensation the voltage at the load point will

improved due to the insertion of the D-STATCOM system. D-STATCOM can

improve the voltage sags since this system can absorb or generate reactive power

easily. The exchange of active and reactive power can be made easily between D-

STATCOM system and the electricity network because the AC output voltage of D-

STATCOM connected directly with the coupling transformer of the electricity

network.

4.4 Conclusion

In this chapter it will discussed about the result and discussion. The result can

be separates into two part that is the result of the simulation without installation D-

STATCOM and the result with installation D-STATCOM.

Fig: 4.10 (B) Voltage at load point 0.9813 Pu for DLG fault

Fig: 4.10 (C) Voltage at load point 0.9858 Pu for SLG fault

68

The distribution system is a 11kV system. When there is a three phase fault

occur at one of the distribution feeder, the voltage sags will take place at the other

feeder. Without installation of the D-STATCOM, the result obtained was as

expected, voltage sags occurred between 0.4 second until 0.6 second. Then, the three

phase fault has been varies with Three Line to Ground (TLG), Double Line to

Ground (DLG), and Single Line to Ground (SLG). The fault resistance also has been

varies from 0.66Ω, 0.76Ω, 0.86Ω and 0.96Ω. Then, from the result it can be observed

that, when the value of fault resistance increase, the voltage also will increased for

different types of fault.

Next, with installation of the D-STATCOM, the result obtained show that the

voltage sag had improved. Almost 90% of the voltage sag during 0.4second to

0.6second back to the nominal value

69

CHAPTER FIVE

CONCLUTION AND RECOMMENDATION

5.1 Conclusion

From this project, it is known that one of the methods to mitigate the voltage

sag (dip) is by using Distribution Static Compensator (D-STATCOM). In order to

show or examine whether voltage sags can mitigate by using D-STATCOM or not,

MATLAB/Simulink was chosen so as to perform simulation for the distribution

scheme and mitigate the voltage sag (dip).

Based on the outcome and simulation that has been completed, it can prove

and verify that D-STATCOM device can be capable to overcome the voltage sags

problem. Although the scheme cannot compensate 100 percent of voltage during

sag, it is an acceptable because the output voltage after compensation still in range of

the nominal value.

The simulation was executed by using the distribution system. In this case,

the fault only takes place at the distribution system and D-STATCOM has a superior

effectiveness because it is deal with distribution system only. Since Distribution

Static Compensator, D-STATCOM is deal with a power electronic device that is

IGBT; automatically there will be a harmonic distortion result. To remove the

harmonics, LCL passive filter can be used.

70

5.2 Recommendation

First Since D-STATCOM deal with a power electronic device, harmonic

distortion can be happen. It is greatly suggested that, LCL passive filter can be

design in MATLAB/Simulink.

Second to get rid of harmonic problem. Use large distribution network. It

recommends that the major circuit of distribution system may be used at least more

than three feeders. So, the effect of fault can be observed at every feeder and how

much percentage of voltage sags took place at every feeder.

Third It is highly recommended to study the voltage sags problem when the

fault happen on the transmission line

Fourth For the future, it is better to make the comparison between two

FACTS device. For instance compare voltage sags mitigation method by using

DSTATCOM and SVC device to observe more clearly about voltage sags

investigation.

71

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75

APPENDIX A

NO ITEM VALUE NOTE

1 Three phase voltage source 230kV/50HZ Wye connection

2 Three phase series RLC

branch

R=0.05

L=0.4806

RL Branch type

3 Logic Fault Time T=0.4-0.6s

4 Universal Bridge IGBT/Diode

Inverter

6 pulse, 3 bridge

arm

Configuration and Specification

76

APPENDIX B

The main circuit of the distribution system without D-STATCOM system

77

APPENDIX C

The main circuit of the distribution system with D-STATCOM system