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Overcurrent and Earth Fault Protection Systems

Assessment

Hossein Askarian Abyaneha,b,c Majid Al-Dabbaghc Mehdi Taleshianb Hossein Kazemi Karegara,b

D.Lidgated M.Janatiane Saeed Amirdashtie

[email protected]

aDepartment of Electrical and Computer Engineering MONASH University, AustraliabDepartment of Electrical and Computer Engineering Amir Kabir University, Iran

cSchool of Electrical and Computer Engineering RMIT University, AustraliadFaculty of Engineering and Computing, Napier University, U.K

eTehran Electricity Company, Iran

Abstract

A new approach for the prediction of Overcurrent (O/C) and earth Fault (E/F) protection system

performance under phase and earth conditions is described in this paper. The approach is ageneralized assessment program being flexible in both system analysis and protection performanceprediction. The first of these enables the effects of mutual coupling between any paralleltransmission lines for a fault in any place is taken into account. The second, however, considers

different types of both O/C and E/F relays characteristics models. The paper concluded by theresults of a study carried out on practical power system networks. These illustrate the applicabilityof the algorithm and the clarity of its output.

Key words- Electrical Engineering, Protection Systems, Assessment

1. INTRODUCTION

The performance of an individual protective relay is

relatively easy to establish on most power systems, asis the performance of a complete protection schemefor say, a single transmission line. However, it

becomes increasingly difficult to predict how differentschemes will interact as the size of the power systemnetwork increases. This is particularly true for

complex interconnected power system networks wherethe failure of relays to detect faults and operate in thecorrect sequence could result in large sections of the

network being deprived of power.

One of the cheapest and simplest forms of power

system protection is provided by the inverse

Overcurrent relay (O/C). Such relays are relativelyeasy to set so that they will protect the system from

short circuit faults in an adjacent component.However, the main advantage of using O/C relays isthat each relay can, by an appropriate choice of

setting, act as the back-up relay to a relay nearer thefault position and operate, after a suitable time delay,to clear the fault in the event of protection or circuitbreaker failure elsewhere in the system. Determinationof the settings of all the O/C relays in a large powersystem network, is itself difficult, although several

techniques, including both ordinary and optimalcomputational procedures, are available [1-6]. The

assessment of the protection performance for such a

system is, however, virtually impossible by manualmeans and temporary alterations to the system, suchas outages for maintenance, cannot be easily taken

into account. Of necessity, therefore, computationalassessment procedures have been proposed [1,7].In an earlier work [8], a computer algorithm was

proposed which would assess the performance of onetype of O/C relays being normal inverse overcurrentprotection systems for both phase and earth faults on

complex interconnected power systems. Thisalgorithm took into account parallel lines andtransformers, teed points, multiple bulk supply points,

directional and non-directional relays andinstantaneous high-set elements. Although, this

algorithm was flexible, it did not take into accountsome very important aspects of power systems and

protection. It did not include a generalized faultanalysis, for example it could not consider mutual

coupling between parallel feeders and for protectionaspect, different types of O/C relays i.e. inverse, veryinverse and extremely inverse for different

manufacturers. The work to be described in this paperis the development of the algorithm to take thesefactors into account.

2. GENERALISED FAULT ANALYSIS

The magnitude of the zero sequence component of themutual impedance between adjacent circuits can be,

typically, 50% of the self-impedance of either circuit


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and should, therefore, be taken into account in earthfault calculations. The positive and negative sequencecomponents of the mutual impedance are, however,

much smaller and can be neglected.

The determination of the zero sequence current pathsfor a network to take account of mutual coupling is, ofcourse, affected by the fault position. In ordinarymethod admittance matrix is considered for short

circuit calculation, mutual coupling is not taken intoaccount. For example if numbers of parallel lines aremore than two, the existing approach does not work.

To solve these, a new method in which mutualcoupling for each case is automatically included isintroduced. Therefore, the existing procedures for

fault calculations are described first, then theweakness of the method for protection predictionpurpose is outlined, and after that, the new fault

analysis method is given.

In the existing method the admittance matrix is made

first and from that, equation (1) is composed [8].

[IT] = [ Ybus] [ VT] (1)

In Equation (1), IT is injection currents vector to the

bus vector, VT bus voltages vector, Ybus admittancematrix. Ybus is made as follows:

ij

jiij

Z

Y1

, = (2)

=

=

n

j

ijii yY1

(3)

where n is number of lines connected to the bus i.

As can be seen, to consider mutual coupling inequations (2&3) when a fault in a parallel line

happened, and for example one of the circuit breakerson the faulty line has tripped, an special arrangementis needed to include relevant mutual coupling. If

number of parallel lines is more than two, againdifferent procedure is needed.

However, in this paper a new generalized approach isproposed which can solve the mentioned problem

easily and automatically. In the method TI matrix is

obtained using different procedures as follows.

a) The first equation is the relation between injectionsand line currents matrix.

mLnmnT ICI = (4)

where

n: numbers of buses,m: numbers of lines,ITn :Injection current matrix.

ILm: Line current matrix.Cnm: Relationship matrix between IT and IL.

b) Second equation expresses the relation between busvoltages and line currents.

mLnmnT IZV = (5)

mTmnmL VZI1

= (6)

where

TV : Differential voltage between two ends

of linesZ : Impedance matrix including lines

impedances and mutual coupling.

For example for two parallel lines with mutual

coupling shown in the Fig.1, the relevant equationwhich include Z bus matrix with mutual coupling isrevealed as equation (7).

Fig.1: Two parallel lines with mutual coupling

For mutual coupling we have:

=

3

2

1

3

2

1

32

21

21

00

0

0

I

I

I

Z

ZM

MZ

VV

VV

VV

(7)

c) Equation (8) expresses the relation between

differential voltage of each line and bus voltages:

1*1*1* nmVmT VCV=

(8)where

V : Voltage matrixVT : Differential voltage matrix

CV : Coefficient matrix.

If IT is substituted from equation (6) into equation (4)

and then VT is substituted from equation (8) into theobtained equation then:

nVmnmmnmInT VCZCI1

= (9)

where

M


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V: Voltage matrixCV, CI: Coefficient matrixZ: Impedance matrix including lines

impedances and mutual coupling

It can be seen all IT matrix elements can be calculatedeasily from equation (9), and from equation (4) alllines currents can be obtained. Of course topologicalchanges and mutual coupling easily can be included in

Z matrix equation as shown in equation (7).

Therefore by comparing equation No. (9) with No. (1)

We will have:

VIbus CZCY1

= (10)

The last equation (10) is an equation in which mutual

coupling has been considered.

It should be noted that if a fault in a parallel lineoccurred, a new bus in the fault point is added. The

faulty line is divided into two new lines and mutualcoupling of each part is considered according to thepercentage of line length in each part automatically inequation (7).

3. PROTECTION MODELLING

A typical inverse time overcurrent relay has two

values to be set, the pickup current value (Ip), and thetime dial setting or time setting multiplier (TDS orTSM). The pickup current value is the minimum

current value for which the relay operates. The time

dial setting defines the operation time (T) of thedevice for each current value, and is normally given asa curve T versus M, where M is the ratio of the relaycurrent, I, to the pickup current.

The relay characteristic can be modelled by the

equation (11) where )(, ipiIIf can be written as

equation (12) [3,5]. This model is an approximate

method and it is not quite precise model.

The model is expressed as:

iipii

iipii

TDSIIfT

or

TSMIIfT

*),(

*),(

=

=

(11)

3

1

2)/(),(

kII

kIIf

k

pii

ipi+

= (12)

Where, Ti, Ii and Ipi are relay operation time, currentflowing the relay and relay current setting. k1, k2 andk3 are constants.

The more precise and flexible model being sachdev

linear model is used in this paper and shown asequation (13) [9]. For different types of overcurrent

relays characteristics the coefficients,410 ,...,, aaa ,

are obtained using curve fitting technique.

4

410 )1/(....)1/(),( +++= iiipi MaMaaIIf (13)

whereMi= Ii/ Ipi

In assessment procedure, the relays are operatedsequentially. Therefore on a real power system, arelay, which eventually operates to trip its circuit

breaker, may have started to operate previously. Thetimes of operation from inspection of the tripping

current may not, be the expected time. Thus, theprevious TSM or TDS will not be valid. This isovercome by calculating a virtual TDS or TSM [7,10].

101 /).( tTSMttTSMNEW = (14)

101 /).( tTDSttTDSNEW = (15)

where

TSMNEW: Virtual TSM

TDSNEW: Virtual TDS

t0: Operation time related to TDS/TMS untilthe current flowing the relay changest1: Total operation time related to the new

fault current with the previous TDS/TMS

4. TESTING PROCEDURE

A flow chart of the computer algorithm developed in

this work is shown in Fig. 2. The input data requiredfor testing procedure are those that would normally berequired to calculate the fault currents for single phase

to earth faults plus the protection system configurationand settings. The main difference in the datarequirements between this algorithm and the algorithmdeveloped in the previous work [7] is that in this case,

the zero sequence components of the mutualimpedances between parallel lines are also required.

As in the previous work, the relay performance settingis carried out in three parts i.e. unit protection failure,

adjacent circuit breaker failure and remote circuitbreaker failure. Firstly, however, the positive,negative, and zero sequence admittance matrices for

the network are composed including, for parallel lines,both self and mutual admittances.


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Primary relay

failure

considered?

Has relay failureat far end

considered ?

Yes

No

No

Yes

No

Yes

No

Yes

Fig.2: Flow chart of the computer algorithm

After composing these matrices, the fault currentflows are calculated according to the equations (4-9).

It should be noted that fault position can be anywhereeither on a bus or any place on a line. The torquedirection for directional overcurrent relays is

computed next. To compute O/C relays operatingtimes, the sachdev model is used according to thesection III. In other word, the sachdev characteristic

model coefficients of any Overcurrent relay being

inverse, very inverse and extremely inverse of anymanufacturer which are previously calculated, stored

in a file. The operating time of the relevant relays arecalculated and the relevant relay settles in the list forthe first selected fault position.

After each circuit breaker trip, the virtual time settings

of all overcurrent relays are calculated according tosection 3 and the procedure continued until the fault isisolated.When the fault is isolated, the algorithm proceeds to

the next fault position. If not, the algorithm performsthe new fault calculation for the new situation. Faultcurrent lines are again calculated and after calculating

relays operating times, the fastest relay is chosen andadded to the list. This continued until fault iscompletely isolated.

This procedure is repeated for all fault positions andall three stages of the assessment procedure for each

fault position.

In this procedure, time for circuit breaker to be fullyopened is considered equal to 0.1 second.

5. EXPERIMENTAL RESULTS

5.1 Norweb Network

A single line schematic diagram of Norweb networkthat has been used to test this algorithm is shown inFig. 3. The network and protection information data

are given in Ref. [8], which are not shown herebecause of pages limitation. The relays installed onthe network are all CDG11 by normal inverse

characteristics and are manufactured by GEC.

For a three fault on line no.3 at 20% of its sending

bus, the relays no. 3 and 23 should operate to isolatethe fault. Whilst Table 1, which shows relayssequence operation, reveals after operation of relays 3and 23, relay no. 11 and 12 being on the generator has

tripped.

It is seen that although fault will be seen by relays no.

3 and 23, however before full opening of their relatedcircuit breakers, two relays on generator feeders will

operate. Therefore, circuit breakers of generatorfeeders are ripped as well as the line circuit breakers.Hence, it can be understood that there is a

miscoordination between generator feeder relays andthe line relays.

The other problem is the total time clearing of faultthat is high, i.e. more than 2 seconds. This clearingtime may cause the power system apparatus to be

damaged.

Start

End

Read input data

Determining main and backup relays

Compose the network configuration

Fault calculation

Calculation of relay operating t imes

Is there any

relay operated?

Change in network due to relay operation

Virtual TSM calculation

Writing results in output

Fault cleared?


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5.2 TEHRAN Electricity Network

Fig. 4 shows 230Kv Tehran Electricity Network. The

system is equipped with different relay characteristics.These characteristics are definite time, normal inverse,

very inverse and long time inverse relays. Some of therelay information showing types, time setting range,and obtained coefficients of Sachdev models are givenin Table 2. Line and other system information are

listed in Ref. [10].Table 3 shows the result of a phase to ground faultoccurred on line no. 34 and at the 5 percent of its

sending bus.

It should be noted that although the values of the

operating times represented in the last column ofTable 3 are relay operating times only, however faultcurrent recalculation and relative operating times are

taken into account after the relevant circuit breakeroperation.

For this fault, relays 36 and 37 must operate to isolatethe faulted line. It is seen that at first, relay number 37operates but then relay 39 and 36 are operated

respectively up to 0.155 seconds. In addition, it is seenthat before full opening of related circuit breaker ofrelay no. 36 i.e. 0.155079 seconds plus 0.1 seconds,

totally 0.255079 seconds, two relays i.e. relaysnumber 58 and 61 are tripped. In other word line no.40 has become unelectrified which is main problem

for co-ordination.

6. CONCLUSIONS

The further development of a generalized computer

algorithm for the assessment of the performance ofovercurrent relays has been described in this paper. Itwas shown that the specific additions to the algorithmare flexibility of the program in both fault analysis and

protection. In fault analysis, it can be highlighted the

capability of dealing with mutual impedance betweenparallel lines. In protection aspect, capabilities ofconsidering different types of relays and modelling

different relays characteristics within the assessmentalgorithm for different power system networks are

illustrated.

7. REFERENCES

[1]. J.P. Whiting and D. Lidgate,Computer prediction of IDMT

relay settings and performance for Interconnected power systems,

IEE Proceeding; Gen, Trans & Distribution; 1983, Vol. 130, No. 3 ,

pp. 139-147.

[2]. Sutherland, P.E., Protective Device Co-ordination in an

industrial Power System with Multiple Sources, IEEE

Transactions on Industry Application, Vol. 33, Issue 4, July-August

1997, pp. 1096-1103.

[3]. A. Urdenata, L. Perez and H. Restrepo, Optimal Co-ordination

of Directional Overcurrent Relays Considering Dynamic Changes in

the Network Topology, IEEE Trans on Power Delivery, Vol. 12,

No. 4, Oct. 1997, pp. 1458-1463.[4]. C.W.So., K.K., Li,Time Co-ordination Method for Power

System Protection Evalutionary Algorithm, IEEE Trans. On

Industry Applications, Sept.-Oct. 2000, pp. 1235-1240.

[5]. C. W. So, K. K. Li, The Influence of Time Coordination

Method on Supply Reliability, IEEE Industry Applications

Conference, 2000, Vol.5,pp.3248.

[6]. H. Askarian Abyaneh, R. Keyhani, Optimal Co-ordination of

Overcurrent Relays in Power System by Dual Simplex Method,

AUPEC 95, The University of Western Australia, Perth, Australia,

1995.

[7]. Lidgate, D., Askarian Abyaneh, H., Computer Assessment of

IDMT Relay Performance for Phase and Earth Faults on

Interconnected power systems, IEE Proceedings, pt.c, Generation,

Transmission and Distribution, Vol. 135, No. 2, March 1989, pp.

157-165.

[8]. H. Askarian Abyaneh, Assessment of IDMT and DistanceRelay Setting, Ph.D Thesis, UMIST, U.K., Oct. 1988.

[9]. IEEE Committee Report, Computer representation of

Overcurrent Relay Characteristics, IEEE Trans. on Power

Delivery, Vol. 4, No. 3, pp. 1656-1667. July 1989.

[10]. M. Taleshian, An Assessment of Various Overcurrent and

Earth Fault Relays Settings on Interconnected Networks, Msc.

Thesis, Amir Kabir University of Technology, Jan. 2001.

Table 1: Relays Sequence Operation for Fault on Line 3

Faulted Bus No Iteration Operating relay No Relay Near Bus Relay far Bus Operating time

12 1 3 1 4 0.9467312 2 23 1 2 1.86066

12 3 11 1 Generator 1.95729

12 4 12 1 Generator 1.95729

* Fault is isolated.

Table 2: Coefficients of Sachdev models of relay TEHRAN network

Relay Manufacturer Type TSM a0 a1 a2 a3 a4

ASEA4 ASAA Very Inverse 0.05-1.1 1.6439 12.827 -10.66 9.2134 -2.566

CDD1 GEC Normal Inverse 0.1-1 1.0427 0.86124 -0.36684 0.13002 -0.014097

CRP9D1 MITSUBISHI Normal Inverse 1-11 0.7243 4.7503 14.474 -4.7714 0.56421

RSAS1 BBC Long Time Inverse 0.1-1 -1.1803 216.26 -258.43 197.08 -50.252

MCGG22 GEC Normal Inverse 0.1-1 1.9302 9.0873 -0.9542 0.15479 -0.007884


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Table 3: Relays Sequence operation for fault on line no. 34

Faulted Bus No IterationOperating relay

NoRelay Near Bus Relay far Bus Operating time

36 1 37 24 28 0.105161

36 2 39 28 24 0.140191

36 3 36 28 24 0.15507936 3 61 15 14 0.196

36 3 58 13 14 0.252

* Fault is isolated.

Fig.3: NORWEB Network

L40 L39

L38

L35

L34

L44

` L43

Fig.4: TEHRAN Network

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