177 SanayePasand Full Paper

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SIMULATION, ANALYSIS AND SETTING OF DISTANCE RELAYS ON

DOUBLE CIRCUIT TRANSMISSION LINES

M . Sanaye-Pasand H . Seyedi

[email protected] [email protected] and Computer Engineering Department

Faculty of Engineering , University of Tehran , Iran

Abstract

In this paper the behaviour of a distance relay on adouble circuit transmission line is analyzed andsimulated using EMTDC/PSCAD software. The

positive and negative sequence couplings between two

circuits of a double circuit transmission line are smalland negligible. However, the zero sequence coupling isnot small. Indeed, if the zero sequence coupling is notcompensated in impedance calculations, distance relaymay either seriously overreach or underreach under

different operational situations. These maloperationsbecome more severe specially in the case that the twonetworks connected to the ends of the protected lineshave extraordinary different equivalent impedances.Under these conditions if the zero sequence mutualcoupling is not compensated, distance relay operation

would not be reliable. However, in the compensatedscheme relay performance is acceptably improved and

the impedance estimated by the relay is close to theactual value. In this paper, a double circuit transmissionline is simulated and operation and setting of lineprotective distance relay is analyzed by simulation.

Mathematical analysis also confirms the results ofsimulations. The same results are also valid for two

transmission lines in close proximity.

Key WordsDouble circuit line protection, distance relay, overreach,underreach

1. Introduction

Two transmission lines on the same tower or in closeproximity have positive, negative and zero sequencemutual couplings. Positive and negative sequencecouplings are acceptably negligible but the zero

sequence coupling is too large to be neglected and inspecial conditions may seriously affect performance of

the distance relay protecting the transmission line [1-7].

Distance relays comprise six internal units to deal withdifferent phase to phase and phase to ground faults

[1,5]. Zero sequence coupling only affects the phase to

ground units, however since most of the transmissionline faults are of this type, this problem deserves specialattention .

Generally, power systems have dynamic structures sothat in different operational conditions some generatorsand transmission lines may either be connected to or

disconnected from the network. So the equivalentimpedances of the two networks connected to the ends

of the protected transmission lines may vary in a widerange [1,3]. Results obtained from both simulations andmathematical analysis show that if the equivalentimpedance of the two networks connected to the double

circuit transmission line are very different from eachother, the impedance estimated by the distance relaywill be much different from the real value.To solve this problem zero sequence current of eachcircuit may be used by the other circuit distance relay tocompensate for the zero sequence coupling when

calculating impedance [1]. Results of simulationsperformed in this paper using a modeled sample powersyetem indicated that in this condition the estimated

impedance is closer to the real value and performanceof the distance relay is much more reliable.In the case that one of the circuits of a double circuittransmission line is out of service and earthed for

maintenance, as the zero sequence current of that circuitis not accessible it is not possible to correct theimpedance calculation with this method. In thissituation, impedance correction requires more completeand advanced methods like pilot protection relaying [3].In this paper, a power system including a double circuit

transmission line is considered and simulated. At thefirst step, a single transmission line is modeled and it isshown that for ground faults, a compensation factor

namely zero sequence factorK0 should be consideredfor correct impedance calculation.In the next step, a double circuit line is modeled and

performance of distance relay is analyzed. Distancerelay performance is evaluated for a wide range ofsystem and transmission line operating conditions.Through simulation studies, the impedance seen by thedistance relay for different conditions is calculated.Mathematical analysis is also performed for impedance

calculation verification. Through these studies,appropriate setting for distance relays could be selected

and analyzed.

2. Distance relay setting

In this paper distance relay setting on a double circuittransmission line including overrreach and underreach


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problems are considered. The impedance estimated bythe relay in steady state condition is calculated and it iscompared with the protective zone of the relay.The first step is to set the relay for a single circuittransmission line. For this purpose primitive simulationsare conducted on a 230 kV single circuit transmissionline with two equivalent networks connected to the ends

of the line. Other system characteristics are specified in

Table 1.

Table 1 : Characteristics of the simulated line

Line length ( km ) 200

Conductor radius ( m ) 0.0203

DC resistance (/km) 0.032

Shunt conductance (/km) 1310

Number of bundles 1

Pos. seq. impedance ( ) 7.5+j101

Zero. seq. impedance ( ) 64+j218

The relay is set at z = 6+j80.8 which is equal to 80percent of the positive sequence impedance of the line.Phase to ground units of the distance relay use voltageand current of the related phase as well as zero sequence

current to estimate the impedance according to equation1 [1] .

In equation 1 , VandI are voltage and current of thefaulty phase respectively and Io is the zero sequencecurrent . K0 is a coefficient which depends on the

positive sequence and zero sequence impedances ( Z1and Z0 ) of the transmission line according to

equation2.

For the line simulated in this paperK0 is equal to

1.77 < -18 .To calculate the voltage and current phasors , thesesignals are sampled with the rate of 800 samples percycle and their magnitude and phase are calculated

using Fourier method [8]. To evaluate setting andoperation of the relay, in three different conditions,

phase to ground faults are simulated. Faults aremodeled at 78% ( 156 kms from the sending end ),75% (150 kms from the sending end ) and 85% (170kms from the sending end ) of the transmission line,

respectively. Table 2 shows the results obtained fromsimulations. As the results show, for the fault inside

the protective zone at 75% the relay trip signal is

activated. Figure 1 shows the locus of the impedanceseen by the relay for this case. Before the fault,calculated impedance is on the remote right hand side

of the impedance plane inside the load area. After the

fault, impedance moves to the left and enters theprotective zone of the relay.

Table2 : Distance relay setting

Fault location

(km) 156 150 170

Actual impedance

()

5.85+

j79

5.6+

j76

6.37+

j86Estimated impedance

for realK0 ()12.86+

j74.2

12.44+

j71.4

13+

j80Estimated impedance

for complexK0 ()5.69+

j79.38

5.48+

j76.19

6.28+

j86.69

Relay trip signal 1 after

60 ms

1 after

33 ms 0

Table 2 also shows that the relay will not trip for anexternal fault at 85% and its trip signal is activatedwith considerable delay for a boundary fault at 78%.For some distance relays K0 is approximated by its

magnitude. It is concluded from Table 2 that ifK0 isconsidered to be a real constant and its argument isneglected a considerable error might be observed in the

Fig.1: Impedance locus for an internal fault

calculated impedance. It is clear that not only the realpart but also the imaginary part of the calculatedimpedance is affected by this error.Figure 2 shows the real and imaginary parts of theimpedance estimated and also the relay trip signal for afault at 85% of the line taking K0 as a real constant

( third column of Table 2 for realK0 ) .In this case a single phase to ground fault occursat t=0.2 s and the resistance and reactance arecalculated by the related phase to ground unit of thedistance relay neglecting the argument ofK0 .As the figure shows the steady state values ofresistance and reactance are 13 and 80 respectively,

and they are considerably different from the actualvalues . It means that neglecting argument ofK0 maycause maloperations specially for faults close to theend of the protective zone .

)1(00 IkI

Vz

+=

)2(1

100

z

zzk

=


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Fig. 2 : Fault at 85% of a single circuit line

3. Double circuit line without zero sequence

compensation

After setting the distance relay as described before,another transmission line with exactly the sameparameters as the first line is installed parallel to it.

Both lines are on the same tower.In this section behaviour of the distance relay setin previous section is analyzed for double circuit line.

For this purpose in 10 different cases the estimatedimpedance of the relay and its trip signal havebeen monitored. Simulation results are shown inTable 3. In Table 3, S1 represents the impedance of

sending end source1 in ohm and S2 is that of receivingend source 2.

Results of the first case of Table 3 are compared withthat of the first case of Table 2. It shows that for a faultat 78% of the line while the equivalent impedances oftwo networks connected to the line are equal,

installing another circuit parallel with the first onemight cause the relay of the faulty line to underreach

for faults next to the end of the transmission line,because the impedance estimated by the relay is morethan its actual value. Results of this case are shown inFig. 3.In case 2, fault occurs at the same position as case1, but the network behind the relay has a less equivalent

impedance than the one in front of it. In this case theestimated impedance is quite similar to that of case 1. Itshows that the effect of reduction of impedance S1 isnegligible in impedance calculation. Case 3 is similarto case 2 but the impedances of both networks have

been increased. In this case the relay seriouslyunderreaches and its trip signal would not be activated

Table 3 : Double circuit line withoutzero sequence compensation

case Faultposition

(Km)

S1()

S2()

Estimatedimpedance

( )

Relay

output

1 156 1 1 7.19+j79.1 1after

80 ms2 156 0.01 1 7.23+j79.13 1after

80 ms

3 156 1 20 8.23+j81.76 0

4 150 1 1 6.91+j75.99 1after

33 ms

5 150 1 10 7.43+j77.19 1after

33 ms

6 150 1 20 7.8+j78.2 1after

57 ms

7 150 1 30 8+j81.2 1after

71 ms

8 150 1 40 8.3+j82.1 1after81 ms

9 150 1 70 8.79+j84 0

10 170 100 1 4.98+j78.57 1after

105ms

Fig. 3 : Fault at 78% of a double circuit line

for a fault at 156 kms from the sending end . Theresults of case 3 are shown in Fig. 4 .It is concluded from cases 2 and 3 that if impedance ofsource 2 is greater than that of source 1 the relayunderreaches. Indeed the greater source 2 impedance is

the more distance relay underreaches. It means thisproblem is more serious for weak networks at therecieving end side of transmission line.


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Cases 4 to 9 show that the underreachingoperation of the relay may prevent it from trippingfor internal faults. In these cases a single phase toground fault is applied at 75% of the line. Theimpedance of the second source is graduallyincreased and it is clear that the more this impedance

increases the more distance relay underreches andfinally in case 9 the relay does not trip for an internalfault .The opposite of the above cases is simulated incase 10. In this case the impedance of the sending endsourceis more than the impedance of the receiving endsource. In this condition the relay seriously

Fig. 4 : Fault at 78% of a double circuit line, S2increased

overreaches and trips for an external fault . The locusof the estimated impedance is shown in Fig. 5 with

two different resolutions .It is concluded from the discussed simulations that

on double circuit lines if the distance relay calculates

impedance using only parameters of its own line, theprobability of maloperations will be high. Thesemaloperations are more serious if the impedances of

two equivalent networks connected to the lineshave great values and subsequently they are weak

networks .

4. Mathematical verification of relay

behaviour on double circuit lines

For a fault on a double circuit transmission line theequivalent positive, negative and zero sequence

networks are shown in Fig. 6.From the equivalent networks and using equation 1 it is

concluded that the impedance seen by the distance relay

Fig. 5 : Locus of the estimated impedance

during overreach operation

during an internal fault is obtained by equation 3 [5].

The fraction inside the bracket is the error in impedancemeasurement .

The parameters of equation 3 are calculated as follows[5] :

m : zero sequence coupling effect factor defined as :

Zm0 is zero sequence mutual impedance of the twocircuits

1

11211

111121

)(2))(1()2( I

ZZZZZnZnI

lss

lssA

++++=

)4()(2

))(1()2(0

000201

0001020 I

ZZZZ

ZZZnZnI

mlss

mlssA

+++

+++=

)3(

1)(2

)(

1

00

1

0

0

1

+++=

kI

I

mI

I

ZnZ

A

A

A

B

lseen

))(1()2(

)1(

000102

0102

0

0

mlss

ss

A

B

ZZZnZn

ZnnZ

I

I

+++

=

)5(1

0

z

zm m=


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Fig. 6 : Equivalent network for fault analysis

Moreover, the parameters of these equations are shownon the figure.Using these equations the estimated impedances for

an uncompensated relay are calculated and the resultsare shown in Table 4.Table 4 confirms the simulation results of Table 3.Small deviations are because of neglecting chargingcurrents and some other simplifications in theoreticalanalysis .

5. Double circuit line using zero sequence

coupling compensation

A suitable method to solve the problems associatedwith the reach of distance relay on a double circuit line

is to consider the effect of zero sequence mutualcoupling. To do this, the effect of zero sequencecurrent of a circuit should be considered whenestimating impedance of the other circuit. It meansthat use of equation 6 instead of equation 1 is moreappropriate in calculation of correct impedance value :

02010A

A

mIIKI

VZ

++= ( 6 )

In this equation let :m : zero sequence coupling effect

I1 : zero sequence current of the protected circuitI2 : zero sequence current of the sound circuitZm0 : zero sequence mutual impedance of the

circuits

Table 4 : Mathematical analysis of uncompensateddouble circuit line

Case Fault

position

(Km)

S1 S2

Estimated

impedance( )

Relay

output

1 156 1 1 6+j79.07 12 156 0.01 1 5.8+j79.19 1

3 156 1 20 7+j82 0

4 150 1 1 5.64+j75.97 1

5 150 1 10 6+j78.5 1

6 150 1 20 6.4+j80 1

7 150 1 30 7+j82 1

8 150 1 40 7.5+j83 1

9 150 1 70 8+j85 0

10 170 100 1 4.96+j76 1

Results of simulations based on equation 6 are shown

in Table 5. Table 5 shows that if equation 6 is usedinstead of equation 1, calculated impedances are veryclose to the actual values. Comparing Tables 2,3 and5 confirms this fact.

Fig. 7 : Estmated impedance locus fo an external fault


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Table 5: Results of simulations for compensateddouble circuit line

Case 1 2 3

Fault

position

156 150 170

S1 1 1 100

S2 20 70 1

Estimatedimpedance

5.3+j77 4.5+j73 8+j87.8

Relay

output

1 after

50 ms

1 after 20

ms

0

Case 1 of Table 5 is exactly similar to case 3 of

Table 3. Comparing the results show that the suggestedmethod has largely prevented the relay fromunderreaching and the estimated impedance is veryclose to the real value. Case 2 of Table 5 is similar tocase 9 of Table 3. It shows that also in this case therelay measurement has been corrected.

Case 3 resembles case 10 of Table 3. In this casecompensation has prevented the relay fromoverreaching. Impedance locus of the estimatedimpedance for this case is shown in Fig.7.It is concluded from the simulations of this section thatmutual zero sequence current compensation is a

relatively suitable method to overcome the underreachand overreach problems of distance relays on doublecircuit transmission lines, so that the zero sequenceeffect and sensitivity to short circuit capacity of thenetworks connected to the protected line is largelyreduced.

6. Analysis of fault resistance effect

One of the problems of distance relays both in singlecircuit and double circuit lines is fault resistance

effect [1]. In this section sensitivity of a double circuitline distance relay to the fault resistance effect issimulated. The distance relay is compensated withK0 and m factors. The results are shown in Table 6 .

Table 6 : Fault resistance effect

Case 1 2 3 4 5

Fault

position

150 150 150 150 150

S1 1 1 1 1 1

S2 1 1 1 1 1

Rf 0 3 10 12 14

Estimated

impedance

6.6+

j76

14+

j74

28+j

70

32+

j68

35+

j67

Relay

output

1 1 1 1 0

Table 6 shows that as the fault resistance ( Rf )increases, real part of the estimated impedance

increases and finally for Rf =14 the relay will not

trip anymore. It should also be noted that the infeedeffect of source 2 influences not only the real partbut also the imaginary part of the calculated

impedance and this problem can also causemaloperations .As simulation results indicate, sensitivity to faultresistance is still a problem even in a compensatedrelay and more complete methods are required to solvethis problem.

7. Double circuit lines when one of the

circuits earthed

In HV substations, transmission line current

transformers are usually installed behind the linedisconnectors and earthing switches, so if one of theparallel lines is earthed its induced current is notaccessible for the distance relay of the other line.Therefore in this situation it is not possibleto compensate the zero sequence coupling effect.

Results of Table 7 show that in this conditionaccording to the impedances of connected networksdistance relay may either underreach or overreach.

Table 7 : Double circuit line when one circuit

is earthed

Case 1 2

Fault

position

160 170

S1 50 1

S2 1 30

Estimated

impedance

11.4+j86 0.95+j79

Relay

output

0 1

8. Simulation of fault at different

locations

In the previous sections, impedance measurement was

mainly performed for faults near the distance relayboundary zone. In this section fault at differentlocations of a double circuit line is simulated andresults for both uncompensated and compensated relaysare evaluated. Simulation results for these two cases arestated in Tables 8 and 9, respectively .

From these tables it is concluded that an uncompensateddistance relay overreaches for faults next to thebeginning of the line, and underreaches forfaults next to the end of the line. So there mustbe a boundary point between these two modes of

operation. At this point even in an uncompensatedrelay the estimated impedance is equal to the actualvalue. According to Table 8 ifZs1 is greater thanZs2the boundary point is next to the end of the line(approximately 170 kms from the sending end) and ifZs2 is greater than Zs1 this point is next to the

beginning of the line (at the first 20 kms of the line).


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Table 8 : results for uncompensated relayFault

locationZseen if

Zs1=10 Zs2

Zseen if

Zs2=10 Zs1

Actual

impedance

20 0.67+j9.4 0.82+j10 0.75+j10

40 1.34+j18.8 1.7+j20.3 1.5+j20.2

60 1.98+j28.2 2.6+30.75 2.3+j30.3

80 2.6+j37.8 3.6+j41.5 3+j40.4

100 3.3+j47.4 4.8+j52.6 3.7+j50.5120 3.9+j57.2 6.2+j64.3 4.5+j60.6

140 4.6+j67 8+j76.7 5.2+j70.7

160 5.5+j78 10.4+j90 6+j80.8

180 7.2+j91 14+j106 6.8+j90.9

Table 9 : results for compensated relayFault

locationZseen if

Zs1=10 Zs2

Zseen if

Zs2=10 Zs1

Actual

impedance

20 0.76+j10.1 0.81+j10.1 0.75+j10

40 1.48+j20.2 1.63+j20.1 1.5+j20.260 2.2+j30.28 2.4+j30.1 2.3+j30.3

80 2.97+j40.4 3.35+j40.2 3+j40.4

100 3.7+j50.53 4.25+j50.2 3.7+j50.5

120 4.4+j60.7 5.1+j60.2 4.5+j60.6

140 5.15+j70.9 6.1+j70.3 5.2+j70.7

160 5.93+j81 7.2+j80.25 6+j80.8

180 6.85+j91 8.36+j90.1 6.8+j90.9

9. Conclusions

In this paper a double circuit transmission line ismodeled and the measured impedance by distance relayin various conditions is studied. According to thediscussed simulations and analysis it is concludedthat :1. Zero sequence mutual coupling may cause the

distance relay to overreach or underreach. Valuesof short circuit capacities of networks connectedto the line have a major effect in this problem.

2. To get accurate distance relay setting it issuggested to take the zero sequence current factorKo as a complex number.

3.

Zero sequence coupling compensation canovercome many of the distance relay problems ondouble circuit lines.

4. Zero sequence compensation can not overcomesome of the intrinsic problems of distanceprotection such as sensitivity toRf.

It i s concluded that the described method is a

relatively suitable method to solve many of theproblems, associated with double circuit lineprotection.

10. References

[1] A. G. Phadke, S. Horowitz,Power system relaying(John Wiley & sons, 1995).

[2] H. Ungrad, V. Narayan , Behavior of distancerelays under earth fault conditions on double

circuit lines (BBC Brown Boveri).[3] H. Ungrad, W. Winkler and A. Wiszniewski

Protection techniques in electrical energy systems,Marcel Dekker, 1995

[4] A. G. Phadke, Jihueng , A new computer basedintegrated distance relay for parallel transmissionlines,IEEE Trans. on PAS, Feb 1985

[5] Protective relays application guide (GEC, 1985)

[6] Y. J. Ahn, S. H. Kang, S. Lee & Y. Kang, Anadaptive distance relaying algorithm immune toreactance effect for double circuit transmissionsystems,IEEE conference, 2001

[7] H. B. Elrefaie, A. I. Megahed, Fault identification

of double circuit lines, Seventh internationalconference on developments in power system

protection, 2001, 287_290[8] A. T. Johns, S. K. Salman,Digital protection for

power systems (IEE, 1995).

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177 SanayePasand Full Paper