2014 INTERNATIONAL CONFERENCE ON COMPUTATION OF POWER, ENERGY, INFORMATION AND COMMUNICATION (ICCPEIC)

978-1-4799-3826-1/14/$31.00©2014 IEEE

Analysis of Unsymmetrical Fault using Symmetrical

Components for the Improvement of

Overcurrent Relay

Akshay V.APG Scholar

K.S.Rangasamy College of Technology,

Tiruchengode, Tamilnadu, India.

join2akshay@gmail.com

Abstract-Transient currents are quite common in power

systems. Transients can occur for both faults and switching

events. In order to reduce hazardous effects of overcurrent

caused by faults, the faster operation of overcurrent protection

is desirable which means maximum sensitivity of the

overcurrent relays. The high sensitivity sometimes causes mal-

trip of relay protection when there is no fault in the system.

This undesirable operation is due to transient events, such as

transformer energizing and induction motors. Therefore,

proper methods must be used to discriminate overcurrent due

to fault from the inrush current due to transformer energizing

and induction motor starting. The main protection system for

a given zone of protection is primary protection. Due to many

factors, in the event of fault a primary protection may fail to

operate and clear the fault gives the importance of backup

protection in the 13 bus radial system. The proposed method

using the concept of symmetrical components for analyzing the

improvement of overcurrent protection in radial system during

the fault and inrush current flowing. This extensive simulation

will be carrying out by using MATLAB/Simulink environment

power system block set toolboxes. The result shows using the

algorithm the new set value of 0.40 could help the overcurrent

relay to discriminate fault from no-fault.

Keywords- Unsymmetrical Faults, Overcurrent relay, Switching

Transients, Symmetrical component.

1. INTRODUCTION

In a power system consisting of generators,

transformers, transmission and distribution circuits, it is

inevitable that eventually some failure will occur

somewhere in the system. When a failure occurs on any part

of the system, it must be quickly detect and disconnect from

the system. There are two principle reasons for it. Firstly, if

the fault is not cleared quickly, it may cause unnecessary

interruption of service to the customers. Secondly, rapid

disconnection of faulted apparatus limits the amount of

damage to it and prevents the effects of fault from spreading

in to the other part of the system.

The detection of a fault and disconnection of a faulty

section or apparatus can be achieved by using fuses or

relays in conjunction with circuit breakers. A fuse performs

both detection and interruption functions automatically but

its use is limited for the protection of low-voltage circuits

only. For high voltage circuits relays and circuit breaker,

Dr. N LoganathanAssociate Professor

K.S.Rangasamy College of Technology,

Tiruchengode, Tamilnadu, India.

loguksrct@gmail.com

which performs the function of circuit interruption. The

protective relay is a device that detects the fault and initiates

the operation of the circuit breaker to isolate the defective

element from the rest of the system.

Overcurrent relay is one of the most important part

of the radial distribution networks[1].In [2] the sensitivity

improvement of overcurrent relay has been studied. The

high sensitivity sometimes causes mal-trip of relay

protection when there is no fault in the system. This

undesirable operation is due to transient events, such as

transformer energizing and induction motors. In [3], effects

of overcurrent on the operation of overcurrent relays due to

these switching have been studied. Therefore, proper

methods must be used to discriminate overcurrent due to

fault from the inrush current due to transformer energizing

and induction motor starting. In [4], the concept of

symmetrical components has been used and a method for

preventing the undesirable relay operations due to over

currents has been presented.

In order to avoid the mal-operation of the relay, the

proposed method based on variation of fundamental

component amplitude of current signal which describes a

criterion function using the concept of symmetrical

components is used for analyzing the improvement of

overcurrent protection in radial system during the inrush

current.

II. STUDY OF DIFFERENT CASES

In this part, the main features of over current due to

different cases (fault, transformer energizing and induction

motor starting) are described.

A. Induction Motor Starting

The starting of a large induction motors leads to a

current typically 5–6 times the rated current. Generally, the

starting current has a very high initial peak, which is

damped out after a few cycles, normally no more than two

cycles depending on the circuit time constant [5], and after

that drops rapidly to a multiple value of its nominal level,

and is maintained during most of the acceleration period.

The current is then smoothly reduced to the nominal value

that depends on the steady-state mechanical load of motor.

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AKSHAY V.A, et.al.: ANALYSIS OF UNSYMMETRICAL FAULT USING SYMMETRICAL COMPONENTS FOR THE IMPROVEMENT

Solid-state starters and variable-frequency drives

(VFD) can keep line current during starting at any preset

value and obviate difficulties of direct across the line

starting [6]. If the utility power system is quite stiff then

direct on line starting will likely be the preferred, which

provides the highest possible starting torque without the use

of a drive, and the shortest acceleration times and most

economical solution for a typical variable-torque load, such

as a pump [7].The high amount of current due to this kind of

starting may cause a trip in a sensitive over-current relays.

The star-delta starting can roughly reduce the starting

currents to 3–4 times the motor full-load current. The

transient inrush current due to the temporary disconnection

of the motor from the supply line in so-called “open circuit

transition” switching procedure, which because of lower

cost is the most common method of star-delta starting,

should be considered. Current surges can typically attain

peak values of up to 20 times the motor full load current

rating and generally last for 10 to about 40 ms . The same

happens in the case of the auto-transformer motor starting.

These transient currents can influence the operation

of highly sensitive over-current relay, particularly when

many switching happen simultaneously. This may be

occurred during energizing a feeder that has not been used

over a long time, which may lead to high starting current

that affects the operation of relays. Therefore, fault current

must be diagnosed from these transient currents.

B. Transformer Energizing

When a transformer is energized, magnetizing inrush

current is expected. The magnitude of this current depends

on the factors such as switching instant, source impedance,

residual flux in the core, transformer size, and design. This

inrush current could reach values as high as 25 times full-

load current and will decay with time until a normal exciting

current value is reached [8]. The decay of the inrush current

may vary from times as short as 20–40 cycles to as long as

minutes for highly inductive circuits, which might have,

hazardous effect on the locally installed over-current relays.

According to one main characteristic of magnetizing current

is that its fundamental component amplitude varies with

time.

C. Faults

The fault analysis of a power system is required to

provide information for the selection of switchgear, setting

of relays and stability of system operation. Faults usually

occur in a power system due to

Insulation failure of equipment

Flashover of lines initiated by a lighting stroke

Permanent damage to conductors and towers or

accidental faulty operations

Faults may either be three – phases in nature

involving all three – phases in a symmetrical manner, or

may be asymmetrical where usually only one or two phases

may be involved. Faults may also caused either by short –

circuits to earth, between live conductors, or by broken

conductors in one or more phases. Sometimes simultaneous

faults may occur involving both both short – circuit and

broken conductor faults (also known as open – circuit fault).

1) Types of Faults

(i) Series fault or open circuit fault

One open conductor fault

Two open conductor fault

(ii) Shunt fault or Short circuit fault

Symmetrical fault or balanced fault

Three phase fault

(iii) Unsymmetrical fault or unbalanced fault

Line to ground (L-G) fault

Line to line (L-L) fault

Double line to ground (L-L-G) fault

A three phase fault is a condition where either all the

three phases of the system are short circuited to each other,

or all the three phases of the system are earthed. This type

of fault is defined as the simultaneous short circuit fault

which occurs at all the three phases and give rise to

symmetrical current. It occurs infrequently, but it is the

most severe type [9].

Thus in general, is a balanced condition and we just

needed to know the positive sequence network to analyze

faults. Typically, only 5% of the initial faults in a power

system are three phase faults with or without earth. Of the

unbalanced faults, 80% are the line – earth and 15% are

double line faults with or without earth and which can often

deteriorate to three – phase fault. Broken conductor faults

account for the rest.

III. PROPOSED METHOD

For any unbalanced or nonsymmetrical network, such

as unsymmetrical fault occurs or having unbalanced load,

symmetrical component conversion can decouple three-

phase system into three independent sequence equivalent

networks, namely positive, negative and zero sequence

network. Therefore these three sequence networks can be

analyzed separately. Then we can convert the sequence

value back into phase variables. This analysis procedure is

commonly used in analyzing the unbalanced system

network, including fault [10]. Symmetrical components can

be viewed as a mathematical tool on which we can entirely

based for analysis of system without converting back to

phase variable. For example, the amplitude of zero sequence

signifies the degree of unbalance, and therefore can be used

to detect the unbalanced fault.

A. Theoretical Background

The symmetrical component transformation for an

arbitrary three-phase set of variables (balanced or

unbalanced), for example, the three-phase current and

inverse transformation is given in (1)

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2014 INTERNATIONAL CONFERENCE ON COMPUTATION OF POWER, ENERGY, INFORMATION AND COMMUNICATION (ICCPEIC)

Here I1, I2 and Io denote the positive, negative and zero

sequence respectively.

In general application in power system analysis, we

typically begin with information in “phase variables”

denoted by subscripts a, b, and c. Note that phase variables

corresponds to actual physical quantities. The value of

converting physical quantities to symmetrical components is

in visualizing and quantization the degree of unbalanced

system network. For a balanced three-phase system, it won’t

be difficult to calculate that the zero and negative sequence

components are zero, and the positive sequence component

is equal to phase a, no matter current or voltage.

Symmetrical components consist of three quantities

[11]: positive sequence (exists during all system conditions,

but is prevalent for balanced conditions on a power system

including three phase faults); negative-sequence (exist

during unbalanced conditions); zero-sequence (exist when

ground is involved in an unbalanced condition). Negative

and zero-sequence components have relatively large values

during unbalanced fault conditions on a power system and

can be used to determine when these fault conditions occur.

Negative sequence components indicate phase-to-phase,

phase-to ground, and phase-to-phase-to-ground faults. Zero

sequence components indicate phase-to-ground and phase -

to-phase-to ground faults.

Also 1+α+α2=0 if currents Ia , Ib and Ic are balanced

(i.e., Ia = I < 0, Ib = I <-1200 and Ic = I <120

0 ). Therefore,

existence of the negative components means that the system

is unbalanced except over a transient period that may be

because of different switching method or non-identical

saturated case of three-phase transformers, three phases are

almost affected simultaneously during switching event.

Consequently, [12] the negative component is not

considerably changed in this case. On the other hand, faults

are classified into symmetrical and asymmetrical parts. The

major feature of these faults is the large value of the

negative component. Therefore, the negative component in

the asymmetrical faults is considerable. For symmetrical

faults the negative component tends to zero. The criterion

function for discriminating fault from non-fault switching is

defined as follows. The criterion function for discriminating

fault from non fault switching is defined as follows:

Since there is a considerable negative component in the

asymmetrical fault case, according to criterion function the

value of R is close to zero. In the switching case, the

negative component is very small and R is close to 1.

In the switching case, the negative component is

very small and R is close to 1. The threshold value of R is

set as R<0.35 indicates the fault; otherwise, over current is

the result of switching. The suggested criterion is based on

the different behavior of the current components during

fault and non fault conditions and is independent of the

amplitude of the current which is advantageous. The reason

is that it operates based on the relative difference between

the negative and positive component of the current. The

suggested criterion function in the asymmetrical distribution

networks also operates properly. During the asymmetrical

fault, the negative component of current increases and the

value of R is much smaller than that before fault event.

Fig.1 Proposed Model

The improvement of overcurrent protection is shown

in Fig1. First, the analog input signal is converted to the

digital signal by a data acquisition unit and entered to the

relay characteristic and detector. If amplitude of the current

becomes larger than the relay setting, the detector unit

operates, if the value of R is smaller than the setting value, it

means that the fault occurs and the output of the AND gate

becomes 1. Now, if in 1 of the AND gate, considering the

relay characteristic, becomes 1, then the relay sends the trip

signal to the circuit breaker.

IV. SIMULATION RESULTS

To show the advantage of the proposed algorithm, the

network parameter of the 13-bus distribution system is

modeled using the MATLAB/Simulink in the Fig.2. In this

13 bus radial system 11kV is the generated voltage and is

stepped up to 11/33kV using step-up transformer for the

transmission and stepped down to 33/11 kV for distribution.

Several non fault events are applied to this system along

with some short circuit events at different times. The

simulation results show that how the proposed algorithm

could help the overcurrent relay to discriminate fault from

non-fault events. The following cases are presented here:

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AKSHAY V.A, et.al.: ANALYSIS OF UNSYMMETRICAL FAULT USING SYMMETRICAL COMPONENTS FOR THE IMPROVEMENT

A. Normal Operation

In order to study different cases, the step down voltage

of 33/11kV is given to the resistive load. The Fig.3 shows the

voltage and current in bus bar 13 during normal operation.

Fig.3 Three Phase Voltage and Current in 13-Bus during

Normal Operation

B. Induction Motor Starting

In order to study an Induction Motor starting, Induction

motor starting is simulated on 13 Bus Distribution System. A

detailed study of a typical case is presented below. In this case

Induction Motor at busbar 13 is switched on at instant t =

2.50sec and three-phase currents are measured. Fig.4 shows

these three-phase currents. As shown in Fig.5, except over a

transient period, R is close to 1 and is larger than setting (0.40)

that shows nonfault case. In this case tripping signal is

prevented.

Fig.4 Three Phase Current due to Induction Motor

Starting

Fig.5 Value of R due to Induction Motor Starting

Fig.2 Simulated 13-Bus Radial System

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2014 INTERNATIONAL CONFERENCE ON COMPUTATION OF POWER, ENERGY, INFORMATION AND COMMUNICATION (ICCPEIC)

C. Fault

i. Phase - Ground Fault

In this case a phase-ground fault occurs at busbar 13

at instant t = 2.5sec and three-phase currents are measured.

Fig.6 shows these three-phase currents. As shown in Fig.9, R

is close to zero (less than 0.40) that shows a fault case in

which the tripping signal is issued.

Fig.6 Three Phase Current due to Phase - Ground Fault

ii. Phase – Phase – Ground Fault

In this case a phase-phase-ground fault occurs at

busbar 13 at instant t = 0.2sec and three-phase currents are

measured. Fig.7 shows these three-phase currents. As shown

in Fig.9, R is close to zero (less than 0.40) that shows a fault

case in which the tripping signal is issued.

Fig.7 Three Phase Current due to Phase – Phase

Ground Fault

iii. Phase - Phase Fault

In this case a phase-phase fault occurs at busbar 13 at

instant t = 2.5sec and three-phase currents are measured. Fig.8

shows these three-phase currents. As shown in Fig.9, R is

close to zero (less than 0.40) that shows a fault case in which

the tripping signal is issued.

Fig.8 Three Phase Current due to Phase - Phase Fault

Fig.9 Value of R due to Fault

C. Simultaneous Induction Motor Starting and Fault

Occurrence

In this case a simultaneous motor starting and phase-

phase fault occurs at busbar 13 at instant t = 2.5sec and three-

phase currents are measured. Fig.10 shows these three-phase

currents. As shown in Fig.11, R is close to zero(less than 0.40)

that shows a fault case in which the tripping signal is issued.

Fig.10Three Phase Current Due to Simultaneous

Induction Motor Starting and Three Phase Fault

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AKSHAY V.A, et.al.: ANALYSIS OF UNSYMMETRICAL FAULT USING SYMMETRICAL COMPONENTS FOR THE IMPROVEMENT

Fig.11 Value of R Due to Simultaneous Induction

Motor Switching and Phase – Ground Fault

In fact, one more advantage of the suggested algorithm

is that, in addition to the diagnosis of the fault in the

individual occurrence from the nonfault case, it enables to

discriminate a fault from simultaneous switching properly.

This is necessary because, if in the case of fault, the operation

of the relay is prevented and it is assumed switching case, it

may lead to a serious damage.

D. Primary and Backup Protection

In the case if unsymmetrical fault occur at bus bar 13 at

instant 2.5sec, the three-phase currents are measured at fault

cases and it shows that these currents are less than 0.40 which

indicates that there is a fault and the relay, R1 trips at instant

2.8sec. If primary protection R1 fails to trip the backup

protection R2 will trip at instant 3.1s and if R1 and R2 fail

Relay R3 will trip at instant 3.4sec is shown in Fig.12, which

gives zones of protection to the complete radial system.

V. CONCLUSION

In this paper, a simple method for improving

overcurrent relays operation has been introduced. The

suggested algorithm is based on the different behavior of the

positive and negative sequence current components during

fault and no-fault conditions and is independent of the current

amplitudes. Based on these differences, a criterion function

has been introduced. The case study was performed using the

simulation software MATLAB and several simulations were

performed in the 13 bus radial system.

The Induction Motor at bus bar 13 is switched on at

instant t = 2.50 seconds the inrush current produced at the

instant except over a transient period, the criterion function R

is close to 1 and is larger than setting (0.40) . During the

unsymmetrical faults injected at instant t = 2.50 seconds, the

criterion function R is close to zero and is less than setting

(0.40). Also simultaneously when induction motor starts and

fault occurs at instant 2.50 seconds, the criterion function R is

close to zero and is less than setting (0.40).

It was concluded that using the algorithm the new set

value of 0.40 could help the overcurrent relay to discriminate

fault from no-fault, considering that undesirable operation of

the overcurrent relays due to the switching is prevented. If

primary protection of relay fails in the 13 bus radial system

the importance of back up protection has been demonstrated

with the new method by simulating various cases during

unsymmetrical faults and induction motor starting.

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2014 INTERNATIONAL CONFERENCE ON COMPUTATION OF POWER, ENERGY, INFORMATION AND COMMUNICATION (ICCPEIC)

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Akshay V.A was born in India in 1986. He

received his B.Tech degree in Electrical

and Electronics Engineering from

Mahatma Gandhi University, Kerala in

2009. He has three years experience in

Industries. He currently pursues Master

Degree in Power Systems Engineering at K.S.Rangasamy

College of Technology, Tamilnadu. His research interests

include Power system Protection, power quality.

Dr.N. Loganathan He was born in India, in

1970. He received B.E. degree in Electrical

and Electronics Engineering in Government

College of Engineering Salem in 1999 and

M.E degree in Power and Energy System

Engineering in University Vishveshwarya

College of Engineering, Bangalore, India and Ph.D. degree in

Anna University,Chennai, India in 2013. Currently he is

working as an Associate Professor at K.S.Rangasamy College

of Technology in the Department of Electrical and Electronics

Engineering, Tamilnadu, India. His research interest include in

High voltage insulation, and power system Engineering.

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