Classification of Voltage Sags in Distribution Systems Due to Short Circuit Faults

Suresh Kamble and Chandrashekhar ThoratGovernment Polytechnic, Aurangabad, Maharashtra-India

stkamble14@gmail.com , csthorat@yahoo.com

Abstract-Voltage sags, one of the major power quality problems, are often caused by power systems faults. Many utilities have been trying to solve this problem but it still has not been any solution to prevent faults in power system. This paper presents a systematic and intuitive approach to analysis and classification of voltage sags, due to short-circuit faults in a 11kV distribution network. The symmetrical component classification is based on the decomposition of the three phase voltages. From positive and negative-sequence voltage a generalized magnitude for the event is defined. Generally, each type of voltage sag has different characteristics, i.e. magnitude and phase angle, and these characteristics are also different when the transformer winding connection is taken into consideration. The industrial 33/11 kV distribution network at Bajaj Hospital is taken as a case study. Simulation studies have been performed by using MATLAB/SIMULINK software.

I. INTRODUCTION

According to IEEE standard 1159-1995, a voltage sag is defined as a decrease in rms voltage down to 90% to 10% of nominal voltage for a time greater than 0.5 cycles of the power frequency but less than or equal to one minute [1]. Voltage sags have always been present in power systems, but only during the past decades have customers become more aware of the inconvenience caused by them [2]. Voltage sag may be caused by switching operations associated with a temporary disconnection of supply, the flow of inrush currents associated with the starting of motor loads, or the flow of fault currents. These events may emanate from the customers system or from the public network. Lighting strikes can also cause voltage sags [3]. The interests in the voltage sags are increasing because they cause the detrimental effects on the several sensitive equipments such as adjustable-speed drives, process-control equipments, programmablelogic controllers, robotics, computers and diagnostic systems, is sensitive to voltage sags. Malfunctioning or failure of this equipment can caused by voltage sags leading to work or production stops with significant associated cost [4] [5].Thus, the automatic classification and characterization of the voltage sags have become an essential requirement for the power quality monitoring.

In the conventional method to assess these effects, voltage sags are characterized by its magnitude and duration. The magnitude is defined as the percentage of the remaining voltage during the sag and the duration is defined as the time between the sag commencement and clearing [5]. However, balanced and unbalanced faults not only cause a drop in the

voltage magnitude but also cause change in the phase angle of the voltage. Therefore, power-electronics converter that use phase angle information for their firing instants may be affected by the phase angle jump [6] [7]. In order to find any solutions for voltage sag problems due to faults, it is necessary to identify characteristics of magnitude, duration, and phase angle variations for the three phases, a voltage type is obtain and two additional characteristics: Characteristic voltage (absolute value of the complex phasor indicating the severity of the sag) and PN factor (defined as the relation between the positive and negative-sequence source impedance). All three characteristics are complex voltages as a function of time. The magnitude of the characteristic voltage is the main characteristics [8].

The scope of the present paper is to study and classify the voltage sags, due to short-circuit faults in actual industrial distribution system. The most commonly used is the ABC classification method while the most theoretical approach is done by the symmetrical component classification method.Therefore, in order to identify the nature of voltage sags in 11 kV distribution network, a significant number of short circuit simulations has been thoroughly performed for all types of faults.

II. METHODS OF SAGS CLASSIFICATION

The two methods for voltage sags classification called as “symmetrical component classification” and “ABC classification”. Both methods have been developed by Professor M.H.J Bollen. The ABC classification is the oldest, simplest and the one most commonly used. However, the classification is based on incomplete assumptions and cannot be directly used to obtain the characteristics of measured sags. The symmetrical component classification is more general and gives a direct link with measured voltages. However, the method is harder to understand and a translation to the ABC classification may be suitable for many applications.

A. The ABC ClassificationThe ABC classification distinguishes between seven types

of three-phase unbalanced voltage sags. Expressions for the complex voltages for these seven types are given in Table I. The complex pre-fault voltage in phase a is indicated by E1.The voltage in the faulted phase or between the faulted phases is indicated by V*. One of the reasons for introducing this classification was to describe the propagation of sags through

257978-1-4673-1653-8/12/$31.00 '2012 IEEE

transformers. Origin and transformation of the seven types are given in Table II and Fig. 1. This method was originally developed to be part of stochastic prediction of voltage sags. From known statistics on faults it is possible to calculate the frequency of occurrence of the different types of voltage sags. This classification can also be used for testing of equipment against voltage sags. By using this classification it is possible to generate the sags that can be expected at the terminals of three-phase equipment. The weak point of this method is that it is only simulation-based. Extraction of the sag type from measured voltage waveforms was not immediately possible. Recent work, however, has shown that the sag type may be estimated in the majority of cases from the three rms voltages only. But this method only works for not too big phase-angle jumps, thus mainly for faults in transmission systems.

TABLE ISEVEN TYPE OF THREE-PHASE UNBALANCED VOLTAGE SAGS ACCORDING TO

THE ABC CLASSIFICATION

B. Symmetrical Component ClassificationThe symmetrical component classification does not suffer

from the same limitation as the ABC classification. The symmetrical component classification distinguishes between sags with the main voltage drop in one phase and sags with the main voltage drop between two phases. Sags due to three-phase faults, with an equal drop in the three phases, are a limit case for both single-phase and two-phase drops. The zero-sequence voltage is treated as a separate characteristic from the beginning and in many studies not even considered. The two other characteristics are the ‘‘characteristic voltage’’ V and the ‘‘PN factor’’ F. The general expression for the ‘‘nonzero-sequence part’’ of a voltage sag with the main drop in phase a (referred to as sag type Da) is:

1 1 32 21 1 32 2

a

b

c

U V

U V jF

U V jF

(1)

The general expression for a voltage sag with the main drop between phases b and c (sag type Ca), is:

1 1 32 21 1 32 2

a

b

c

U F

U F jV

U F jV

(2)

Where F V . Note that a balanced sag, due to a three-phase fault, is obtained for F=V. Similar expressions hold for the other four sag types: Db (drop in phase b), Dc (main drop in phase c); Cb (main drop between phases a and c); Cc (main drop between phase a and b). The somewhat illogical notation was used to make the classification consistent with the older ABC-classification. For F = E1 and V = V*, (1) and (2) are identical to type D and type C, respectively, as defined for the ABC classification in Table I. The definition of characteristic voltage and PN-factor as well as the algorithm for obtaining the characteristics from measured voltage wave shapes are both based on symmetrical components.

TABLE IISAGS AT DIFFERENT VOLTAGE LEVELS DUE TO DIFFERENT FAULT TYPES

III. FAULT ANALYSIS DUE TO SYMMETRICAL COMPONENTCLASSIFICATION

A. Three-Phase FaultConsider a three-phase fault somewhere in a three-phase

power network. If the fault-impedance is zero, the remaining voltage is zero at the fault location and nonzero but less than the pre-fault voltage for all other locations in the network. To

Type Voltages Type Voltages

A

*

* *

* *

1 1 32 21 1 32 2

a

b

c

U V

U V jV

U V jV

E

1

* *

* *

1 1 32 21 1 32 2

a

b

c

U E

U V jV

U V jV

B

*

1 1

1 1

1 1 32 21 1 32 2

a

b

c

U V

U E jE

U E jEF

*

* *1

* *1

1 1 1 32 3 61 1 1 32 3 6

a

b

c

U V

U V E V j

U V E V j

C

1

*1

*1

1 1 32 21 1 32 2

a

b

c

U E

U E jV

U E jV G

*1

* *1

* *1

2 13 3

1 1 1 33 6 21 1 1 33 6 2

a

b

c

U E V

U E V jV

U E V jV

D

*

*1

*1

1 1 32 21 1 32 2

a

b

c

U V

U V jE

U V jE

Fig. 1. Sags at different voltage levels due to different fault type

Fault type Measurement locationBUS1 BUS2 BUS3

Three-phase A A ATwo-phase-to-ground E F GTwo-phase C D CSingle-phase-to-ground B C D

258

determine the retained voltage at a given location in the network, the point-of-common coupling between the fault and the sag location is determined. The retained voltage at the point-of-common coupling is obtained from:

111

1 1

F

S F

ZU E

Z Z (3)

U1 is the (positive-sequence) retained voltage at the pcc; E1 is the pre-fault voltage at the pcc; ZF1 is the (positive-sequence) impedance between the pcc and the fault position; ZS1 is the source impedance at the pcc. Expression (3) gives a direct relation between the retained voltage and the fault location. This expression is the basis for the characterisation of voltage sags due to non-symmetrical faults. The fault impedance is normally assumed zero in voltage-sag calculations. Including it requires adjusting the value of the impedance to the fault to include the fault impedance.

B. Phase-to-Phase FaultIn case of non-symmetrical faults, not only positive-

sequence quantities need to be known like in (3), but also negative-sequence quantities, index 2, and zero-sequence quantities, index 0. See Fig. 2 for a definition of the various quantities used below. For a phase-to-phase fault between the phases b and c, the following expressions are obtained for positive-sequence and negative-sequence component of the retained voltage:

11 1

1 2 1 21 S

S S F F

ZU E

Z Z Z Z (4)

22 1

1 2 1 2

S

S S F F

ZU E

Z Z Z Z (5)

To characterise the event, the sum and difference of positive and negative-sequence voltage are used:

1 21 2 1

1 2 1 21 S S

S S F F

Z ZU U E

Z Z Z Z(6)

1 21 2 1

1 2 1 2

F F

S S F F

Z ZU U E

Z Z Z Z (7)

Note that (7) is similar to the expression for the retained voltage during a three-phase fault, (3). The difference is that the sum of positive and negative-sequence impedance determines the retained voltage. Due to this similarity (7) is used as a generalisation of the retained voltage. Expression (7) is referred to as the ‘‘characteristic voltage’’. Expression (6) in turn is referred to as the PN factor as its value is determined by the difference between positive and negative-sequence source impedance.

In most cases positive-sequence and negative-sequence impedance are equal. This holds especially for the impedance between the pcc and the fault and to a lesser extends for the source impedance at the pcc. Under these assumptions, the characteristic voltage is the same for a three-phase fault as for a phase-to-phase fault, and the PN-factor becomes equal to

the pre-fault voltage. This event is referred to as a type Ca-sag, i.e. the main drop in voltage is in the phases b and c, with characteristic voltage according to (7) and PN-factor according to (6). For a phase-to-phase fault between phases a and b the resulting component voltages at the pcc are:

11 1

1 2 1 21 S

S S F F

ZU E

Z Z Z Z (8)

22

2 11 2 1 2

S

S S F F

a ZU E

Z Z Z Z (9)

with 1 1 32 2

a j a rotation over 120º in the complex

plane. The resulting voltage sag is of type Cc (main drop between phases a and b) with characteristic voltage:

1 21 2 1

1 2 1 2

F F

S S F F

Z ZU aU E

Z Z Z Z (10)

and PN factor

1 21 2 1

1 2 1 21 S S

S S F F

Z ZU aU E

Z Z Z Z(11)

The difference is not in the value of the characteristics but in the way they are calculated from the positive and negative-sequence voltage. For a fault between phases a and c the result is a sag of type Cb with characteristic voltage:

1 221 2 1

1 2 1 2

F F

S S F F

Z ZU a U E

Z Z Z Z (12)

and PN factor

1 221 2 1

1 2 1 21 S S

S S F F

Z ZU a U E

Z Z Z Z(13)

Fig. 2. Voltage-divider model for three-phase unbalanced voltage sags.

C. Two-Phase-to-Ground FaultFor a fault between phases b, c and ground, positive and

negative-sequence voltage at the pcc are as follows:

Source

PCCU1 U2

Impedance to the faultZF1 ZF2 ZF0

Source voltage E1Source impedance ZS1 ZS2 ZS0

Faulted feeder

Load or monitor

Fault

259

1 0 0 2 21 11

S S F S FZ Z Z Z ZU E

D(14)

2 0 02 1

S S FZ Z ZU E

D (15)

with0 0 1 1 2 2

1 1 2 2

S F S F S F

S F S F

D Z Z Z Z Z Z

Z Z Z Z (16)

Comparing this with the results for the definitions of the basic sag types in Table III shows that this is a sag of type Ca with the following characteristics:

1 2 0 0 1 2 211

S S S F S S FZ Z Z Z Z Z ZV E

D(17)

1 2 0 0 1 2 211

S S S F S S FZ Z Z Z Z Z ZF E

D(18)

The characteristic voltage is again a measure of the fault location, thus of the severity of the sag. This can best be seen by considering equal positive and negative-sequence impedance: ZS1 = ZS2 and ZF1 = ZF2, after which (17) and (18) are simplified into:

11

1 2

F

F S

ZV E

Z Z (19)

11

1 1 0 01

2S

S F S F

ZF E

Z Z Z Z

(20)

The expression for the characteristic voltage is again the same as for three-phase and phase-to-phase faults. The difference is only in the value of the PN factor which is less than the pre-fault voltage E1 but larger than the characteristic voltage V*.The other sag types from Table III are again obtained for faults between other phases and after transfer of the sag through a transformer.

D. Single-Phase-to-Ground FaultConsider a single-phase fault in phase a at the fault location

in Fig. 2. The resulting positive and negative-sequence voltages at the pcc are obtained from:

11 1

1 2 0 1 2 01 S

S S S F F F

ZU E

Z Z Z Z Z Z(21)

22 1

1 2 0 1 2 0

S

S S S F F F

ZU E

Z Z Z Z Z Z (22)

Using the definitions from Table III characteristic voltage and PN factor are obtained from the following expressions (note that this is a Da event):

1 21

1 2 0 1 2 01 S S

S S S F F F

Z ZV E

Z Z Z Z Z Z(23)

1 21

1 2 0 1 2 01 S S

S S S F F F

Z ZF E

Z Z Z Z Z Z(24)

The characteristic voltages for sags due to single-phase and due to phase-to-phase faults, introduce the voltage in the faulted phase at the pcc in Fig. 2.

1 2 01

1 2 0 1 2 01 S S S

S S S F F F

Z Z ZV E

Z Z Z Z Z Z(25)

(Note that this is the same voltage as indicated by V* in the expression for sag type B in Table I.) The voltage in the faulted phase at the pcc can be obtained from the same single-phase voltage-divider model as for three-phase faults, but with the sum of positive, negative and zero-sequence impedance. Together with (23) this results in:

0 1 21

1 2 0 1 2 0

S S S

S S S S S S

Z Z ZV E V

Z Z Z Z Z Z(26)

The characteristic voltage contains a non-zero component that is independent of the fault position. Even for a fault at the pcc (V* = 0) a non-zero characteristic voltage results. Also note that (26) is only influenced by the source impedance at the pcc. The voltage sag due to a single-phase fault is less severe than due to a phase-to-phase fault at the same location. However, when the fault location is not known, a sag due to a single-phase fault cannot be distinguished from a sag due to a phase-to-phase fault. This justifies the use of the same classification for sags due to single-phase and phase-to-phase faults [9] [10] [11].

TABLE IIIDEFINITION OF CHARACTERISTICS FOR THREE-PHASE UNBALANCED VOLTAGE

SAGS

IV. SIMULATION MODEL

Fig. 3 shows the single line diagram of the express feeder for Bajaj Hospital under study. It is fed from 33/11 kV distribution substation of Maharashtra State Distribution Company Limited (MSEDCL), Railway station, Industrial area, Aurangabad, India has been consider for voltage sag classification. There are two numbers of 5MVA identical Power Transformers each 33/11 kV Delta/star connected with

Types Drops in Phases Characteristic Voltage

PN Factor

Ca bc U1 - U2 U1 + U2

Cb ac U1 - a2U2 U1 + a2U2

Cc ab U1 - aU2 U1 + aU2

Da a U1 + U2 U1 - U2

Db b U1 + a2U2 U1 - a2U2

Dc c U1 + aU2 U1 - aU2

260

Sr. No Component Details1 Source 10 MVA, 33 kv, X/R=10

2 Transformer T1

5 MVA, 33/11 kv, %Z=7.15, X/R=10, DYn11

3 Transformer T2

750 KVA, 11/0.415 kv, %Z=5, X/R=6, DYn11

4 Line (0.6748+j0.372) impedance per km5 Load 190 kw and 130 kvar6 Frequency 50 Hz

solidly grounded neutral in substation. T1 is the 5MVA, 33/11 kV, 50 Hz power transformer in substation and T2 is the 750 KVA, 11/0.415 kV, Delta/star distribution transformer. The 11 kV overhead feeders are of type dog conductors and of length 2 km. The system is modeled using the simulink and SimpowerSystem utilities of MATLAB.Table IV shows system parameters used in the simulation.

The performance study of sample system is carried out for classification of voltage sags due to power system faults. It is assumed that a fault has occurred at position F, on the primary side of distribution transformer T2, and the fault lasted for 4 cycles from t = 0.045 to 0.125 seconds. The monitoring equipment is installed at the bus1 and bus2.

TABLE IVSYSTEM PARAMETERS

V. SIMULATION RESULT

A. Three-Phase-to-Ground FaultThe balanced voltage sags are caused by three phase faults

anywhere in the distribution system. Balanced sags do not

change when transferred through a transformer, it is referred as type A. Fig. 4(a) shows the three phase voltage waveform at the bus1, since it is balanced fault, voltages in the three phases are same. The phase angle jumps in the faulted phase voltage are shown in Fig. 4(b). In distribution systems, the source impedance is typically formed by a transformer with a rather large X/R ratio, whereas the impedance to the fault is formed by lines with a much smaller X/R ratio. This will lead to a significant change in phase angle. All the phase voltages decrease to zero, and all voltages change their phase angle jumps are -107.7, +132.5 and +12.56 degree for phases a, b and c respectively. For the balanced faults, there will be no negative and zero-sequence components in the phase voltages measures at bus1, which can also be seen in Fig. 4(c), the positive-sequence voltage decreases to zero, and remains almost constant during the fault.

The voltage sags due to three-phase fault, the characteristic voltage and PN factor will be equal to the voltage in any of in the affected phases, both the value decrease to zero as shown in Fig. 4(d).

Fig. 4. Three-phase-to-ground fault (a) Voltage waveforms, (b) Phase-angle jump, (c) Sequence voltages, (d) Characteristic voltage (solid line) and PN-factor (dashed line)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-1.5

-1

-0.5

0

0.5

1

1.5

3-ph-ground voltage wave form

Time (sec)

Volta

ge (p

u)VaVbVc

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

-100

-50

0

X: 0.1025Y: -107.7

Phase-angle jump-3-ph-fault

Phas

e-an

gle-

jum

pPh

ase

A (d

eg)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-200

0

200

X: 0.1059Y: 132.5

Phas

e-an

gle-

jum

pPh

ase

B (d

eg)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20

50

100

X: 0.1073Y: 12.56

Time (sec)

Phas

e-an

gle-

jum

pPh

ase

C (d

eg)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20

0.2

0.4

0.6

0.8

1

Phase-sequence-3-ph-fault

Time (sec)

Vol

tage

(pu) Positive

NegativeZero

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18-0.2

0

0.2

0.4

0.6

0.8

1

Time in seconds

Volta

ge in

pu

VcharPN

(a)

(b)

(c)

(d)

Fig.3. Single line diagram of distribution system for simulation

F

Load

Waluj33 kV

33 kV

11 kV

5MVA 33/11 kV

Rahul -Nagar

MIT MIDCKancha-nwadi

ITI

OverheadLine

BUS1

BUS2

T2750KVA 11/0.415kV

T110MVA 33/11 kV

261

B. Single-Phase-to-Ground FaultMost of the faults on the utility transmission and

distribution systems are single-phase-to-ground faults. These faults are the most common cause of voltage sags for industrial customers. The voltage on the faulted phase drops to zero at the fault location, whereas the other two phase voltages remain more or less unchanged. This results in unbalanced voltage sag between phases. This imbalanced voltage sag is then transferred down to the equipment terminals within the industrial system. The sag experienced by the equipment is different to that observed on the utility network or service entrance due to the influence of the load in the industrial distribution system [12].

Consider a single-phase-to-ground fault on phase a, at position F and sag is monitor at the bus1 as shown in Fig.3. The phase a voltage will drop to zero and the phases b and cwill remains unchanged as shown in Fig. 5(a). According to the classification, this is a sag type B or Da. During the sag the voltage in the faulted phase a is suppressed with a large phase-angle jump (+145) degree, whereas the phase angle jump in the other two non-faulted phases is almost not affected as shown in Fig. 5(b). In terms of symmetrical components, we see a drop in positive-sequence voltage decrease to 0.66 pu and rise in negative and zero-sequence voltages. The magnitudes of negative and zero-sequence voltages are similar (0.33pu) as shown in Fig. 5(c). The characteristic voltage (V) and PN factor (F) are shown in Fig. 5(d). The sag type is Da with characteristic voltage, PN factor and zero sequence are V = 0.33 pu, F = 1 and Vo = 0.33 pu.

Removing the zero-sequence component from type B sag or transferring through DY11 connection transformer results in a type C sag. Type C sag is characterized by a major voltage drop in one phase a and drop in the non-faulted phase b. In this case the voltage drop in phases a and b is same 0.44 pu and phase c is 0.76 pu as shown in Fig. 6(a) and Fig. 6(b).The positive and negative and zero-sequence voltages are 0.507 pu, 0.2547 pu and zero in Fig. 6(c) respectively. However the values of the characteristic voltage and PNfactor at bus2 are 0.7617 pu and 0.2523 pu as shown in Fig. 6(d).

C. Phase-to-Phase FaultA phase-to-phase fault (type C sags) appeared between

phases a and b on position F and monitoring equipment is installed at bus1 as shown in Fig. 3. A phase-to-phase fault leads to a voltage drop between the faulted phase a and b is 0.5 pu and phases c will remains unchanged at bus1 as shown in Fig. 7(a) and Fig. 7(b), including a phase-angle jump for phase a decrease to -30o to -90o, phase b increase -150o to -90o and phase angle jump in the phase c is almost not affected as shown in Fig. 7(c). This is a sag type Ca according to symmetrical component classification, with characteristic voltage V = 0, PN factor F = 1 and zero-sequence Vo = 0 as shown in Fig. 7(d).

D. Two Phase-to-Ground FaultType E voltage sag occurs for a two-phases-to-ground fault

between phases b and c at F. The voltage sag was recorded at the same location (bus1). The Fig. 8(a), Fig. 8(b), shows the three phase voltage waveform, rms voltage and phase-angle jump. This shows a significantly large drop in voltages in the faulted phases b and c, but no change phases a. The phase voltage b and c drops to zero in magnitude, and phase-angle jump are (-46) and (-40) degree for phase b and c respectively as shown in Fig. 8(c). The three-sequence voltage of the two-phase-to-ground fault is shown in Fig. 8(d). The positive-sequence voltage is decrease to 0.3295 and rise in Negative and zero-sequence voltages is 0.3295 pu respectively. The value of the sag characteristics voltage decrease to zero and PN factor is 0.658 pu for type E sag as shown in Fig. 8(d).Phase-to-phase fault (type C sags) appeared between phases a and b.

Fig. 5. Single-phase-to-ground fault (a) Voltage waveforms, (b) Phase-angle jump, (c) Sequence voltages, (d) Characteristic voltage (solid line) and PN-factor (dashed line)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-1.5

-1

-0.5

0

0.5

1

1.5

3-ph-voltage-SLGF-bus1

Time in seconds

Volta

ge in

pu

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18-150

-100

-50

0

50

100

150Phase-angle jump-SLGF-bus1

Time in seconds

Phase

-angle

jump

(deg

)

PhaseAPhaseBPhaseC

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

0

0.2

0.4

0.6

0.8

1

X: 0.09563Y: 0.659

Sequence voltage- SLGF- bus1

Time in seconds

Volta

ge in

pu

X: 0.09648Y: 0.3285

PositiveNegativeZero

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180

0.2

0.4

0.6

0.8

1

X: 0.0957Y: 0.328

Vchar-PN-SLGF-bus1

Time in seconds

Volta

ge in

pu

PNVchar

(a)

(b)

(c)

(d)

262

HELPFUL HINTS

Fig. 6. Single-phase-to-ground fault (a) Voltage waveforms, (b) rms voltage, (c) Phase-angle jump, (d) Sequence voltages, (e)Characteristic voltage (solid line) and PN-factor (dashed line)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

13ph-voltage-SLGF-bus2

Time in seconds

Vol

tage

in p

u

VaVbVc

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

X: 0.09727Y: 0.4399

Rms voltage-SLGF-bus2

Time in seconds

RMS

volta

ge in

pu

Va-rmsVb-rmsVc-rms

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

-150

-100

-50

0

50

100

150

Phase-angle jump-SLGF-bus2

Time in seconds

Phas

e-an

gle j

ump

(deg

)

PhaseAPhaseBPhaseC

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

X: 0.09617Y: 0.2547

Sequence voltage-SLGF-bus2

Time in seconds

Vol

tage

in p

u

X: 0.09547Y: 0.507

PositiveNegativeZero

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

X: 0.09922Y: 0.2523

Vchar-PN-SLGF-bus2

Time in seconds

Volta

ge in

pu

X: 0.09406Y: 0.7617

VcharPN

Fig. 7. Phase-to-phase fault (a) Voltage waveforms, (b) rms voltage, (c) Phase-angle jump, (d) Sequence voltages, (e) Characteristic voltage (solidline) and PN-factor (dashed line)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

-1.5

-1

-0.5

0

0.5

1

1.5

3ph-voltage-LLf-bus1

Time in seconds

Vol

tage

in p

u

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180

0.2

0.4

0.6

0.8

1

1.2

X: 0.0957Y: 0.4949

RMS voltage -LLF-bus1

Time in seconds

RMS

volta

ge in

pu

VaVbVc

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18-200

-150

-100

-50

0

50

100phase-angle-LLF-bus1

Time in seconds

Phas

e-an

gle j

ump

(deg

)

PhaseAPhaseBPhaseC

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

0

0.2

0.4

0.6

0.8

1

Sequance voltage-LLF-bus1

Time in seconds

Vol

tage i

n pu Positive

NegativeZero

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18-0.2

0

0.2

0.4

0.6

0.8

1

Vchar-PN-LLF-bus1

Time in seconds

Vol

tage i

n pu

PNVchar

(a)

(b)

(c)

(d)

(e)

(a)

(b)

(c)

(d)

(e)

263

VI. CONCLUSION

This paper presents a case study of voltage sag classification due to short circuits faults in a distribution network. The software MATLAB/SIMULINK has been applied to practical 11 kV distribution feeder at Bajaj Hospital. The classification of voltage sags can be very useful for power electronic loads; a classification using positive and negative sequence voltages is more useful for induction motor loads.

The symmetrical component classification is a systematic approach it covers all cases and is therefore preferable above the ABC classification, which is a more intuitive approach.The symmetrical component classification does not suffer from the same limitation as the ABC classification. The symmetrical component classification distinguishes between sags with the main voltage drop in one phase and sags with voltage drop between two phases.

The symmetrical component method characteristics voltage and PN factor, The characteristic voltage is almost the same for sags due to three-phase, two-phase-to-ground and phase-to-phase faults, at the same location. Future work includes the effect of sag types on different load and sag characteristic should be investigated.

REFERENCES

[1] “IEEE Recommended Practice for Monitoring Electric Power Quality,” IEEE Std. 1159-1995, June 1995.

[2] Pirjo Heine and Matti Lehtonen, “Voltage sag distributions caused by power systems faults,” IEEE Transactions on Power Systems, vol.18, No.4, pp.1367-1373, November 2003.

[3] Raj Naidoo and Pragasen Pillay, “A New method of voltage sag and swell detection,” IEEE Transactions on Power Delivery, vol.22, No.2, pp.1056-1063, April 2007.

[4] Elisa Espinosa Juarez and Araceli Hernandez, “An analytical approach for stochastic assessment og balanced and unbalanced voltage sags in large systems,” IEEE Transactions on Power Delivery, vol.21, No.3, pp.1493-1500, July 2006.

[5] Dong-Jun Wan, Sean-Ju Ahn and Yop Chung, “A new definition of voltage sag duration considering the voltage tolerance curve,” IEEE Bologna Power Tech. Conference, Bologna, Italy, vol. 4, June 23-26, 2003.

[6] M.H.J. Bollen, “Understanding Power Quality Problems: Voltage Sags and Interruptions,” New York, IEEE Press, 2000.

[7] Emmanouil Styvaktakis, Math H.J. Bollen and Irene Y.H.Gu, “Automatic classification on power system event using rms voltage measurements,” IEEE Power Engineering Society Summer Meeting, vol.2, pp. 824-829, July 2002.

[8] Magnus Ohrstrom and Lennart soder, “A comparison of two methods used for voltage dip characterization,” IEEE Power Tech. Conference, Bologna, Italy, vol. 4, June 23-26, 2003.

[9] M.H.J. Bollen, L.D. Zhang, “Different methods for classification of threephase unbalanced voltage dips due to faults,” Electric Power Systems Research, Vol.66, pp. 59-69, July 2003.

[10] M.H.J. Bollen and Irene Y.H.Gu, “Signal Processing Of Power Quality Disturbances,” IEEE Press, 2006.

[11] Bollen, M. H. J. and Styvaktakis, S., “Characterization of Three-phaseUnbalanced Sags, as easy as one, two, three,” IEEE PES SummerMeeting 2000, Seattle, WA, USA, July 2000.

[12] Gülali Yalçinkaya, Math H. J. Bollen, and Peter A. Crossley, “Characterization of voltage sags in Industrial distribution systems,” IEEE Transactions on Industry Applications, Vol.34, No.4. pp. 682-688, July/August 1998.

Fig. 8. Two-phase-to-ground fault (a) Voltage waveforms, (b) rmsvoltage, (c) Phase-angle jump, (d) Sequence voltages, (e) Characteristicvoltage (solid line) and PN-factor (dashed line)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

-1.5

-1

-0.5

0

0.5

1

1.5

3ph-voltage-LLG-bus1

Time in seconds

Volta

ge in

pu

VaVbVc

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

0

0.2

0.4

0.6

0.8

1

1.2rm s voltage-LLG-bus1

Time in seconds

RM

S vo

ltage

in p

u

Va-rmsVb-rmsVc-rms

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

-150

-100

-50

0

50

100

phase-angle jump-LLG-bus1

Time in seconds

Phas

e-an

gle

jum

p (d

eg) PhaseA

PhaseBPhaseC

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

0

0.2

0.4

0.6

0.8

1

sequence voltage-2ph-ground-bus1

Time in seconds

Volta

ge in

pu Positive

NagativeZero

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

0

0.2

0.4

0.6

0.8

1

Vchar-PN-LLG-bus1

Time in seconds

Vol

tage i

n pu

VcharPN-factor

(a)

(b)

(c)

(d)

(e)

264