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Analysis of Distance Relay Trip Regions forEHV and UHV transmission lines

Kurre Ravishankar, D ThukaramDepartment of Electrical EngineeringIndian Institute of Science, Bangalore

Email: {ravishankarkurre,dtram }@ee.iisc.ernet.in

Abstract This paper presents comparative evaluation of thedistance relay characteristics for UHV and EHV transmissionlines. Distance protection relay characteristics for the EHV andUHV systems are developed using Electromagnetic Transients(EMT) program. The variation of ideal trip boundaries for boththe systems are presented. Unlike the conventional distance pro-tection relay which uses a lumped parameter model, this paperuses the distributed parameter model. The effect of larger shuntsusceptance on the trip boundaries is highlighted. Performance of distance relay with ideal trip boundaries for EHV and UHV lines

have been tested for various fault locations and fault resistances.Electromagnetic Transients (EMT) program has been developedconsidering distributed parameter line model for simulating thetest systems. The voltage and current phasors are computed fromthe signals using an improved full cycle DFT algorithm taking 20samples per cycle. Two practical transmission systems of Indianpower grid, namely 765 kV UHV transmission line and SREB24-bus 400kV EHV system are used to test the performance of the proposed approach.

I. INTRODUCTION

Digital relaying has been developed for more than fortyyears since Rockefeller conceptualized a single computerperforming all the relaying functions in a substation [1]. A

distance relay calculates the impedance to the fault using themeasured local voltage and current. The calculation has beencarried out in difference equations [2] or using phasors ob-tained by Fourier algorithms [3]. Methods exploiting travelingwave components to achieve ultra high speed protection havealso been proposed [4]. Even though a distance relay doesnot locate a fault, it is required to determine the reach of itssetting. It will determine whether a fault is within or outsidethe intended protection zone. For a zone-one distance relay,it is usually expected that its transient measurement error of impedance be less than 5% around the reach of its setting [5].Therefore the accuracy of a line model is also important fora distance relay in spite of its difference to a fault locating

device. The model accuracy, like impedance calculation, isparticularly important or faults that are close to the boundaryof the protection zone, especially for zone-one protection.

Recently, adaptive transmission relaying concepts becamean attractive possibility due to the rapid developments incomputer and cummunication technologies [6]. Relatively in-expensive computera for substation control, data acquisitionand on-line analysis are now available while on-going de-velopment of large bandwidth communication systems, using

new technologies such as bre- optics, are linking togetherall modes of utility operation [7]. In keeping with this trendadaptive protection achieves better performance from a pro-tection system by allowing the settings to be made withconsideration for fewer contingencies than is presently thecase [8]. Fewer constraints on a problem leads, in general,to a solution which is closer to the ideal. Sophisticated digitalrelays can be designed to adapt to varying conditions, viz,be free from the limitations imposed by the need to treatchanging network conditions as unknown constraints. Previousstudies considered the accuracy of the impedance calculation.For instance, the errors associated with transient voltage andcurrent sampling windows were analyzed. It was shown thatthe accuracy decreases as the window becomes shorter andhence the calculation speed is higher. As a result, the minimumwindow size can be specied. Improved performance wasobtained by using an algorithm to adjust the boundary angle.However, if the system conditions vary in a wide range andfaults occur through high arc resistances the relay may loseselectivity.

There is little reported research work on the effect of

distance relay trip region due to variation in transmissionline parameters of EHV/UHV systems. Conventional distancerelays usually employ a lumped parameter line model and thetotal line impedance is basically the series impedance per unitlength multiplied by the length. The fault impedance capturedby the relay is also interpreted based on such a simpliedmodel. A lumped parameter line model has been adequate inmany cases but its suitability is questionable for a long UHVtransmission line where the distributed shunt capacitance mayresult in large errors, deteriorating the relay performance. Inthe literature relay performance for UHV line has not beenstudied with ideal trip boundaries, due to which maloperationcan occur.

This paper presents relay settings for digital distance relaysunder different operating conditions.The relay can operatefaster and be more sensitive to various faults under differentconditions without loosing selectivity. An accurate faultedtransmission line model which considers distributed shuntcapacitance has been presented. The relay trip boundaries arecomputed considering transmission line model under realisticfault conditions. An accurate trip region has been computedfor two practical transmission systems of Indian power grid. It

978-1-4577-1510-5/11$26.00 c 2011 IEEE

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is observed trip boundaries of 765kV UHV transmission linecompared to 400kV EHV line are much different in naturebecause of high shunt capacitance. An improved full cycleDFT algorithm to estimate and eliminate the decaying DCcomponent in fault current signal is used [9]. Both systemsare simulated using the developed Electromagnetic Transientsprogram [10].

II . D ISTANCE RELAY SETTING FUNDAMENTALS

A digital distance relay uses sampled voltage and currentdata from the relaying point for measuring the apparentimpedance and then uses an appropriate characteristic to makeproper decisions to disconnect a faulted line. With referenceto Fig. 1, suppose a distance relay is installed at S and afault occurs at Q through a fault resistance Rf ,, the apparentimpedance of measured by the relay at S can be expressed as

Z s relay = Z sq +I sq + I rq

I sqRF (1)

In the above conventional reactance-type measurement us-ing the a.c. quantities available at S only it is impossible todetermine the infeed current I r q owing through the fault re-sistance. The remote-end infeed is dependent not only on faultlocation and fault resistance but also on source impedance of the two ends. To avoid maloperation, in conventional distancerelay setting, including digital distance relaying practice, asafety margin is used in zone 1.

ZsqZ s

IsqI rq

Z rqR

f

Z r

E rE s Q

S R

Fig. 1. fault model of transmission linel without considering shunt capaci-tance

Apparent impedance seen by the relay at S in terms of pre-fault load current and E S andE R can be written as

I load = I sq 0 =(E s E r )

(Z s + Z r + Z l )(2)

V q0 = E s (Z s + Z sq )I load (3)

I f =V q0

(Z qq + Z f )= I sq (F ) + I rq (F ) (4)

I sq (F ) =(Z rq + Z r )

(Z s + Z r + Z l )I q(F ) = K 1I f (5)

where

K 1 =(Z rq + Z r )

(Z s + Z r + Z l )(6)

V s (F ) = Z sq I sq + R f E s (Z s + Z sq )(Z pq + R f )

(E s E r )(Z s + Z r + Z l )

(7)If E S = 1 and E R = h exp( j ) are voltage potentials then

Z s relay = Z sq +R f

(Z qq + R f )(1 h exp( j ))(Z qq + Z rq )+( Z s + Z sq )h exp( j ) + K 1

= Z sq +R f

(Z qq + R f )K n + K 1= Z sq + Z

(8)

Z qq =(Z r + Z rq )(Z s + Z sq )

Z t(9)

Z t = ( Z s + Z r + Z l ) (10)

Where Z s and Z r are source impedances Z l is the lineimpedance, angle between E S andE R , and h is ratio of E Rto E S

R f is will change with type of fault which is given infollowing section.Right handside second term in the Eq.(8) Z causes the traditional relay to overreach or under reachunder different system conditions.Here voltages and currentsare considered to be vectors and Rf is matrix Electrical powerengineers have dreamed for many years of setting the distancerelay to respond to faults in the whole line immediately. Anadaptive setting scheme can almost make this true [7].

III. FAULTED TRANSMISSION SYSTEM MODEL

A. An accurate faulted transmission line model

The transmission system model adopted for system analysis

under fault condition is shown in Fig. 2. The shunt capacitanceeffects are considered Rf is general fault with a fault resis-tance. E S andE R are the equivalent potentials pd is distancefrom the bus S . d is the length of transmission line [11].

E s

Zs

I s I R

Rf

V s ZR

VR

ER

c

Z csinh(rpd)

c

Z csinh(r(1p)d)Relay

F

1 / z

t a n

h ( p

r d / 2 )

1 / z

t a n

h ( ( 1

p

) r d / 2 )

Fig. 2. Accurate EHV/UHV transmission line model under fault condition

B. A general fault model

A general fault resistance model shown in Fig. 3 is adoptedin this paper. The conductance matrix for the fault circuit asin Fig. 3is

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f g bc g ab

f

f

g acgc gbf

ga

a

bc

f f

Fig. 3. A three phase fault model

Gaf bc =

gaf + gabf + gacf gabf gacf gabf gbf + gabf + gbcf gbcf gacf gbcf gcf + gcbf + gacf

(11)where g represents conductance. which yields to the faultmodel for all single phase to gound, phase to phase and3-phase to ground faults by taking proper correspondingconductances

I abcF = Gabcf V

abcF (12)

Where V abcF and I abcF are fault voltages and currents Fig. 3.

IV. IMPROVED DFT ALGORITHM TO REMOVE DC OFFSETFROM CURRENT

Full-cycle DFT lters are among the most popular inrelaying. For current waveform i(t) = A sin (t + ), thefundamental frequency components are provided by [3].

I C =2N

N 1

k=0

ik cos(2kN

) (13)

I S =2N

N 1

k=0

ik sin(2kN

) (14)

where N is samples per cycle

phasor is I = I C + jI S

However, the DFT is not immune from the DC componentand the decaying DC component in the fault current can causeundesirable oscillations in the phasors. These oscillations cancause abnormal operation of protection system as well asinaccurate fault location.

Various Fault transient studies carried out for 765 kVtransmission systems [12] showed that the wave shape of faultcurrent signicantly varying for the fault inception instant andalso the peak magnitude of fault currents are signicantlylarge in UHV (765 kV) systems as compared with EHV (400kV) systems. So estimation of DC offset in current signalis challenging when fault occurs on UHV transmission linebecause time constant and magnitude will depend on power

system conguration at the moment of the fault and alsolocation of that fault on the line. It is a well established factthat line relays have a tendency to overreach in the presenceof DC offset components in fault current waveforms. In faultlocation, the decaying component therefore has to be removedfrom these waveforms. A number of techniques have beenattempted to deal with such situations [13] In this paper animproved algorithm is presented to estimate and remove thedecaying DC components in the fault current signal. Thefault current consisting of exponentially decaying component,fundamental and harmonic components can be expressed as

i(t) = I 0e t/ +

k= n

k=1

I k sin(kt + k ) (15)

Where I 0 is the magnitude of the decaying DC offset, and isthe time constant, k is the harmonic order, I k is the magnitudeof the kth harmonic component, k is the phase angle of thekth harmonic component, is the fundamental frequency andn is the maximum harmonic order.

The integral of the second term of Eq. (15) over oneperiod(T) is zero and the integral of the rst term is thedecaying DC component [9].

t

t T i(t)dt =

t

t T I 0e

t/ dt

= I 0 e t/ (1 eT/ )

= f (t) (16)

After simplifying equation (16) we get time constant andmagnitude of the DC component as follows

1

= (1 f (t + t)

f (t))

1 t

(17)

I 0 =f (T )

(e T/ 1)(18)

By subtracting the calculated DC value from each of thesampled data in buffers which contain one cycle of sample datawe can extract fundamental component exactly by applyingDFT.

V. SIMULATION RESULTSThe proposed faulted system model applied to typical UHV

and EHV Indian systems for computing the trip boundaries of distance relay

A. 24 bus equivalent EHV system of Indian Southern Grid An EHV Equivalent of 400kV 24-bus system, part of Indian

southern grid, is considered for study. The single line diagramof the system is shown in Fig. 4. The system has 4 generators16 transmission lines and 4 transformers . It has four genera-tors with a total generation of 2684.94MW, 1046.3MVARand total load of 2620MW, 980MVAR . The transmissionline 18 22, of typically 290km is considered for protectionstudies. The a-g faults for these cases assumed to occur at0.040s. Fault resistance RF is varied from 1.6 (0.001 p.u) to

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~

~

~

~1

15

16

5

10

18

24

3

17

2

11

12

14

8

13

22

1921

7

20

23

9

6

Fig. 4. Single line diagram of 24 bus EHV system of Indian southern grid

160 (0.1 p.u). Fault location is varied from 0 to 95% from bus

18. All currents and voltages are expressed in per unit valueswith a base MVA of 100 and Base voltage of 400 2/ 3kV .Primary voltage and current signals at bus 18 observed by therelay at bus 18 are shown in Fig. 5 and Fig. 6 for a-g faultat a distance of 5% from bus 18 for fault resistance of 1.6 Current signal at bus 22 is shown in Fig. 7 Phasors from thecurrent and voltage signals are computed by using improvedfull cycle DFT [9] taking 20sampples per cycle. The apparentimpedance seen by relay at bus- 18 as time progreses within25ms are computed.

0 0.05 0.1 0.15 0.21.5

1

0.5

0

0.5

1

1.5

v o

l t a g e

i n p u

time in sec

vavbvc

Fig. 5. Voltage at bus 18 for a-g fault at a distance of 5% from bus 18 andR f =1.6

As shown in Fig. 8, for base case condition, four boundarylines are obtained by varying R f and fault location by EMTPsimulations. An ideal trip region in R-X plane is obtained

Boundary I is for different fault locations for solid faults.Boundary II is fault nearer to bus 18 for R f varying from0 to 160 . Boundary III is with R f = 160 and varyingfault locations from 0 to 95% of the line. Boundary IV is forfault at 95% distance and varying fault resistance from 0 to160. Fig. 9 shows the impedance trajectories for various faultresistances and fault locations.

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.24

2

0

2

4

6

8

10

12

14

c u r r e n t

i n p u

time in sec

iaibic

Fig. 6. Current in line 18 22 for a-g fault at a distance of 5% from bus18 and R f =1.6

0 0.05 0.1 0.15 0.210

5

0

5

10

15

20

c u r r e n t

i n p u

time in sec

iaibic

Fig. 7. Current in line 22 18 for a-g fault at a distance of 5% from bus18 and R f =1.6

0 0.05 0.1 0.15 0.2 0.25 0.3 0.350.02

0

0.02

0.04

0.06

0.08

0.1

X p

u

R pu

II 95%

III Rf=0.1 pu

I Rf=0 pu

II 0%

Fig. 8. Relay characteristics for 24 bus EHV test system

0.1 0 0.1 0.2 0.3 0.4 0.5 0.60.2

0.15

0.1

0.05

0

0.05

0.1

0.15

R pu

X p

u

RX1

RX2

RX3RX4

RX5

Rf=0.16ohm

Rf=160ohm

p=0

p=0.95

Fig. 9. Relay characteristics for 24 bus EHV test system

B. 765 kV typical Indian UHV system

The test system considered for studies is a 765kV typicalIndian transmission system connected between Anpara-Unnaoof UPSEB. Equivalent system diagram is shown in Fig. 10.

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Sending end bus(bus 3) is located at Anpara and Receivingend bus (bus4) is located at Unnao. Initially the system isassumed to be operating in steady state balanced conditionand delivering a load of 600MW and 200MVAR . Theinitial voltages at the buses are obtained from AC load owsolution. The generator side source strength is considered tobe 5000MV A and load side source strength is consideredto be 4000MV A. Faults simulated at ctitious bus 7 forvariable distances ( p ranging from 0.02 to 0.95) from thereceiving end bus 4. Results for only selected few cases arepresented in this paper. The faults for these cases assumed tooccur at 0.040s. Fault resistance RF is varied from 5.852 (0.001 p.u) to 585.2 (0.1 p.u). The line parameters areL1 = 0 .0008944H/km, L 0 = 0 .002692H/km, C 1 =1.639 10 8F/km, C 0 = 1 .3 10

8F/km . All currents andvoltages are expressed in per unit values with a base MVA of 100 and Base voltage of 765 2/ 3kV .

~G

4321

15 kV 400kV 765kV 765kV 400kV 15kV

6(Subsration S)ANPARA

5

UNNAO (substation R)

409km

200MVAR 200MVAR

~ ~

Fig. 10. Test system of 765kv Indian transmission system

Primary voltage and current signals at bus 3 observed by therelay at bus 3 are shown in Fig. 13 and Fig. 11 for a-g faultat a distance of 95% from bus 3 for fault resistance of 5.8 .Fig. 12 and Fig. 14 shows the current and voltage signals atbus 3 for fault resistance of 58.2 at a distance of 95% fromBus 4.

0 0.05 0.1 0.15 0.215

10

5

0

5

10

15

20

25

30

c u r r e n t

i n p u

time in sec

iaibic

Fig. 11. Current in line 3 7 for a-g fault at a distance of 95% from bus3 and R f =5.8

Fig. 15 and 16 give the apparent resistance and reactancesobserved by the relay at bus 3. As shown in Fig. 17, for basecase condition, four boundary lines are obtained by varyingRf and fault location by EMTP simulations. An ideal tripregion in R-X plane is obtained. Boundary I is for differentfault locations for solid faults. Boundary II is fault nearer tobus 3 for R f varying from 0 to 250 . Boundary III is withR f = 250 and varying fault locations from 0 to 95% of theline. Boundary IV is for fault at 95% distance and varyingfault resistance from 0 to 250. Fig. 18 shows the ideal tripregions for different loading conditions mentioned in Table

0 0.05 0.1 0.15 0.220

15

10

5

0

5

10

15

20

25

c u r r e n t

i n p u

time in sec

iaibic

Fig. 12. Current in line 3 7 for a-g fault at a distance of 95% from bus3 and R f =58.2

0 0.05 0.1 0.15 0.21.5

1

0.5

0

0.5

1

1.5

v o

l t a g e

i n p u

time in sec

vavbvc

Fig. 13. Voltage at bus 3 for a-g fault at a distance of 95% from bus 3 andR f =5.8

0 0.05 0.1 0.15 0.21.5

1

0.5

0

0.5

1

1.5

v o

l t a g e

i n p u

time in sec

vavbvc

Fig. 14. Voltage at bus 3 for a-g fault at a distance of 5% from bus 3 andR f =58.2

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20.1

0.08

0.06

0.04

0.02

0

0.02

0.04

0.06

0.08

0.1

time(sec)

R &

X ( p

u )

Fig. 15. Effective resistance and reactance observed by the relay located atbus 3 for a a g fault at 95% from bus 3 and fault resistance R f = 5 .8

I. Fig. 19 shows the impedance trajectories for various faultresistances and fault locations.

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0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20.1

0.08

0.06

0.04

0.02

0

0.02

0.04

0.06

0.08

0.1

time(sec)

R &

X ( p

u )

RX

Fig. 16. Effective resistance and reactance observed by the relay located atbus 3 for a a g fault at 95% from bus 3 and fault resistance R f = 58 .2

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.080.05

0.04

0.03

0.02

0.01

0

0.01

0.02

0.03

0.04

X p

u

R pu

II 95%

Iii Rf=0.1 pu

I Rf=0 pu

Iv 0%

Fig. 17. Relay characteristics for 765 kV test system

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.090.06

0.05

0.04

0.03

0.02

0.01

0

0.01

0.02

0.03

0.04

X p

u

R pu

70% base case

130% base case

base case

Fig. 18. Relay characteristics for 765 kV test system under different loadingconditions

0.02 0 0.02 0.04 0.06 0.08 0.10.1

0.08

0.06

0.04

0.02

0

0.02

0.04

R pu

X p u

0.02 0 0.02 0.04 0.06 0.08 0.10.1

0.08

0.06

0.04

0.02

0

0.02

0.04

R pu

X p

u

RX4

RX2RX3

RX5

RX6

RX1

RX7

before t=0.04sec

after t=0.065sec

Fig. 19. Relay characteristics for 765 kV test system

VI. CONCLUSION

In this paper the distance relay characteristics for UHV andEHV transmission lines are developed using ElectromagneticTransients (EMT) simulations. UHV distance relay character-istics are seen to be much different from that of EHV system

TABLE IVARIOUS LOADING CONDITIONS CONS IDERED FOR RELAY

CHARACTERISTICS

Load E 1 E 6

600+ j 153 0.9533 7 .46 0.99949 27 .567 780+ j 69 0.9724 9 .545 1.022 35 .9586

420+ j 214 0.94 5 .295 0.974 19 .2688

because of high shunt capacitance of long UHV line. Thevariation of ideal trip boundaries for both the systems arepresented. Unlike the conventional distance protection relaywhich uses a lumped parameter model, the distributed param-eter model is used. The effect of larger shunt susceptance onthe trip boundaries is presented. Performance of new distancerelay has been tested for various fault locations and faultresistances. A modied full cycle DFT algorithm eliminate thedecaying DC component in fault current is used to computeaccurate current phasor. The proposed approach performancehas been tested on two practical transmission systems of Indianpower grid, namely 765 kV UHV transmission line and SREB24-bus 400kV EHV system are used The trip boundariescomputed under different conditions will useful for EHV/UHVdigital relay trip decisions.

REFERENCES[1] G. Rockefeller, Fault protection with a digital computer, Power Appa-

ratus and Systems, IEEE Transactions on , no. 4, pp. 438464, 1969.[2] B. Mann and I. Morrison, Digital calculation of impedance for trans-

mission line protection, Power Apparatus and Systems, IEEE Transac-tions on , no. 1, pp. 270279, 1971.

[3] A. Phadke and J. Thorp, Computer Relaying For Power Systems . JohnWiley & Sons, 1988.

[4] L. Jie, S. Elangovan, and J. B. X. Devotta, Adaptive travelling waveprotection algorithm using two correlation functions, IEEE Transactionson Power Delivery , vol. 14, no. 1, Jan. 1999.

[5] S. Horowitz, A. Phadke, and J. Thorpe, Adaptive transmission systemrelaying, Power Delivery, IEEE Transactions on , vol. 3, no. 4, pp. 14361445, 1988.

[6] J. Thorp, A. Phadke, S. Horowitz, and J. Beehler, Limits to impedancerelaying, Power Apparatus and Systems, IEEE Transactions on , no. 1,pp. 246260, 1979.

[7] Y. Xia, K. Li, and A. David, Adaptive relay setting for stand-alonedigital distance protection, Power Delivery, IEEE Transactions on ,vol. 9, no. 1, pp. 480491, 1994.

[8] Z. Xu, S. Huang, L. Ran, J. Liu, Y. Qin, Q. Yang, and J. He, A distanceprotection relay for a 1000-kv uhv transmission line, Power Delivery, IEEE Transactions on , vol. 23, no. 4, pp. 17951804, 2008.

[9] Y.-S. Cho, C.-K. Le, G. Jang, and H. J. Lee, An innovative decaying dccomponent estimation algorithm for digital relaying, IEEE Transactionson Power Delivery , vol. 24, no. 1, pp. 7378, Jan. 2009.

[10] K. Ravishankar and D. Thukaram, Manual Electromagnetic TransientsProgram . Department of Electrical Engineering, Indian Institute of Science, Bangalore, India, 2009.

[11] J. Izykowski, R. Molag, E. Rosolowski, and M. Saha, Accurate locationof faults on power transmission lines with use of two-end unsynchro-nized measurements, Power Delivery, IEEE Transactions on , vol. 21,no. 2, pp. 627633, 2006.

[12] D. Thukaram, K. Ravishankar, A. Kumar, and S. Kolla, Switchingand fault transient analysis of 765 kv transmission systems, in Power Systems, 2009. ICPS09. International Conference on . IEEE, 2009, pp.16.

[13] T. S. Sidhu, X. Zhang, F. Albasri, and M. S. Sachdev, Discretefourier-transform-based technique for removal of decaying dc offset from phasorestimates, Proc. Inst. Elect. Eng., Gen.., Transm. Distrib , vol. 150, no. 6,pp. 745752, Nov. 2003.