A Novel Backup Distance Protection Scheme for Series-Compensated Transmission Lines

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 2, APRIL 2014 699

A Novel Backup Distance Protection Scheme forSeries-Compensated Transmission Lines

S. M. Hashemi, M. Tarafdar Hagh, Member, IEEE, and H. Seyedi

Abstract—Conventional distance protection applies the posi-tive-sequence impedance to protect a line against short-circuitfaults. Series-compensated lines may considerably change thepositive-sequence impedance of the fault path and cause thedistance relays to maloperate. To overcome this problem, themutual impedance between phases of a transmission line is usedfor the design of a new distance protection scheme. The voltagesand currents of both ends of the line are applied to compute themutual impedance between the relay and the fault point. Theproposed scheme has a reliable performance in protection againstsingle-phase and double-phase-to-ground faults and, therefore, itcan be used as a backup protection. Simulation results approvethe efficiency of the proposed method in protection of series-com-pensated transmission lines.

Index Terms—Distance protection, mutual impedance, seriescompensation, transmission line.

I. INTRODUCTION

S ERIES compensation is a common practice to increase theloadability of transmission lines and improve the power

system stability. Series capacitors are used to compensate a por-tion of inductive impedance of the line and provide a bettervoltage profile along the transmission line [1]. Distance relaysprovide the main protection of transmission lines, as well as in-terconnected distribution networks [2]. These relays measurethe positive-sequence impedance to the fault and compare itwith their predefined characteristic. The presence of series ca-pacitor in the fault loop affects reach and directionality of dis-tance relays. The reach of distance relays may experience twomajor problems: 1) the reduction of series inductance of linewhich shifts down the locus of fault impedance in the R-X plane,and decreases the reliability of relay and 2) subsynchronous res-onance which may introduce remarkable delays in the responseof digital phasor estimation methods [3].Directionality of the distance relay in series-compensated

lines may fail due to voltage or current inversions. A commonsolution for this problem is using memory voltage as thereference quantity to detect fault direction [2].

Manuscript received February 20, 2013; revised May 31, 2013; accepted July02, 2013. Date of publication July 30, 2013; date of current version March 20,2014. Paper no. TPWRD-00212-2013.The authors are with the Faculty of Electrical and Computer Engineering,

University of Tabriz, Tabriz 51666-15813, Iran (e-mail: [email protected]; [email protected]; [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TPWRD.2013.2272765

The literature shows that protection of series-compensatedlines has been an interesting issue for protection engineers andhas been the subject of many works for several decades. Ef-fects of series compensation on the performance of directionalcomparison relaying, as well as phase comparison protection,are analyzed in [4]. A modified procedure is presented in [5]for the protective zone setting of distance relays in series-com-pensated lines. In [6], the problems of distance protection in se-ries-compensated lines are explained in detail. Series capaci-tors are commonly protected against transient overvoltages bymetal–oxide varistors (MOVs). The nonlinear function of MOVis considered in [7] and [8] to modify the distance protectionby computing the voltage drop along the series compensator.This voltage drop is also used in [9], as well as some additionallogics, to increase the accuracy and speed of the first zone ofdistance relays. In [10], some modification techniques are pro-vided for pilot protection schemes to prevent malfunctions ofthe main line and its adjacent line relays. Problems associatedwith the directionality of protective relays in series-compen-sated lines are addressed in [11] where a new process of direc-tional relaying using the phase difference between the prefaultand postfault currents is presented. In [12], the residual currentis applied to compensate for the impedance measurement errorin MHO relays.Proposed protective schemes for series-compensated line are

not limited to the distance protection. Differential protection, inthe form of the phase comparison scheme, is expected to op-erate properly in series-compensated lines [13]. Some paperspropose protection schemes using the transient features of cur-rents and voltages during fault conditions. These schemes can bemainly divided into the traveling wave-based [14]–[17] and su-perimposed-based methods [18], [19]. Despite high-speed per-formance, these schemes are dealing with two challenges: 1)If, for any reason such as measurement errors, the current andvoltage samples of the transient time after fault inception are notproperly achieved, the schemes cannot operate correctly. 2) Inthe case of slowly evolving faults, superimposed voltages andcurrent may graduate so slowly that the required traveling wavesare not generated.This paper proposes a new distance protection, based on the

mutual impedance between phases. Applying the voltages andcurrents of both ends of the line, the proposed scheme can pro-tect the line against single-phase and double-phase-to-groundfaults. Since themutual impedance is not affected by series com-pensation, the proposed scheme can provide a reliable backupprotection for series-compensated transmission lines. The restof this paper is organized as follows: Section II analyzes the ef-fects of series compensation on the mutual impedance between

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700 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 2, APRIL 2014

Fig. 1. Simple transmission line.

phases. Section III explains that how the mutual impedance iscomputed in the proposed scheme. In Section IV, the character-istic of the proposed relay is introduced. The proposed protec-tive algorithm and simulation results are presented in Sections Vand VI, respectively. Finally, Section VII contains the conclu-sion of this paper.

II. EFFECTS OF SERIES COMPENSATION ON THE

MUTUAL IMPEDANCE

Considering Fig. 1, the following equations can be written ina three-phase system:

(1)

In (1), , , and are the self impedances of phases , ,and , respectively. Moreover, the mutual impedances betweeneach of these two phases is represented by , , and .The matrix representation of (1) may be provided by

, where is

(2)

Assuming that all three-phase conductors are the same andthe line is fully transposed, it can be stated that

and . Therefore, (2) isrewritten as

(3)

The sequence impedance matrix of is denoted byand is expressed as

(4)

where , , and are known as zero-, positive-, and nega-tive-sequence impedances, respectively, and is equal to

(5)

Fig. 2. Series-compensated transmission line.

Equation (4) implies that the zero-sequence and posi-tive-sequence impedances are given byand , respectively. So the mutual impedancebetween phases is

(6)

Now, suppose that a series compensator with the impedanceis located at the beginning of the transmission line (Fig. 2).

Applying relations similar to (1)–(4) yields

(7)

(8)

From (8), the zero- and positive-sequence impedances (i.e.,and , respectively) are given by

(9)

(10)

It is obvious from (10) that the presence of the series com-pensator changes the value of the positive-sequence impedance.Therefore, this impedance is not competent for relaying. How-ever, the mutual impedance does not differ from its value in thenoncompensated lines and yet it can be computed by (6).

III. COMPUTATION OF MUTUAL IMPEDANCE FORDISTANCE RELAYING

In the proposed method, the mutual impedance is computedusing (6). However, before using this equation, the zero-se-quence and positive-sequence impedances should be calculated.For this purpose, a series-compensated transmission line is con-sidered, as shown in Fig. 3(a). In this figure, the proposed relayis located at the bus , and protects the line . Meanwhile, itis assumed that the voltage and current phasors measured in theremote bus (bus ) are available in the local bus (bus ) usingsynchronous measurements and high-speed communicationbetween the two buses. For a single-phase-to-ground fault inthe phase , the sequence circuits are depicted in Fig. 3(b).

HASHEMI et al.: A NOVEL BACKUP DISTANCE PROTECTION SCHEME FOR SERIES-COMPENSATED TRANSMISSION LINES 701

Fig. 3. (a) Series-compensated transmission line. (b) Corresponding sequencecircuit for a single-phase-to-ground fault.

Using this figure, the following equations can be derived. Itshould be noted that in the following equations, the subscripts0, 1, and 2 stand for zero-, positive-, and negative-sequencecomponents, respectively

(11)

(12)

In (12), is the positive-sequence impedance to faultwhich is measured by the relay and is given by

(13)

where . Hence, , where. Therefore, (11) can be rewritten as

(14)

Hence, we have

(15)

Similarly, it can be concluded form (12) that

(16)

Fig. 4. Characteristic of the proposed relay.

Subtracting (15) from (16) yields

(17)A comparison between (6) and (17) reveals that the mutual

impedance between phases of line is achieved by

(18)

IV. CHARACTERISTIC OF THE PROPOSED RELAY

The computed mutual impedance, similar to the positive-se-quence impedance, is proportional to the distance between therelay and fault point. Therefore, the concept of the protectivezone in conventional distance protection can be applied to theproposed protective scheme. This means that the first zone maycover 85% of the main line, the second zone can cover the mainline plus 50% of the next shortest line, and the third zone maycontain the main line plus the next longest line plus a 25%margin. Since the nature of mutual impedance between phasesis mainly inductive, a characteristic similar to the reactance re-lays can be used for the proposed relay. A typical characteristicis represented in Fig. 4. This characteristic is only for the firstzone, where is equal to 85% of the mutual reactance ofthe main protected line. Characteristics of the second and thirdzone can be easily set according to their reaches.It should be noted that by restricting zone 1 to 85% of the line

length, the proposed relay is unable to protect the entire line in-stantaneously, and 10 to 15% of the line end will be protected inzone 2 with a definite time delay. This time delay is unavoidabledue to some errors in instrument transformers and inaccuraciesin the value of line parameters. For providing an instantaneousprotection for the entire line, communication between the localand remote relays can be established to apply a pilot protectionscheme, such as the directional comparison blocking (DCB).Thus far, it is assumed that the fault has no resistance. For the

computation of mutual impedance in the previous section, using(13) was needed, which is applied in conventional distance pro-tection. However, this equation neglects the fault resistance. Thepresence of fault resistance may considerably change the valueof (13) and, therefore, makes the computed mutual impedancedeviate from its correct value. For analyzing the effect of faultresistance on the performance of the proposed relay, two condi-tions can be considered, as follows.


Fig. 5. Effects of fault resistance when the power-flow direction is forward.(a) A typical line with an internal fault. (b) A typical line with an external fault.(c) Modification of the relay characteristic in internal faults. (d) Modification ofthe relay characteristic in external faults.

A. Power-Flow Direction is Forward

Before explaining this condition, it should be noted that pro-tective relays of transmission lines are directional. It means thatthey can discriminate between faults occurring in front of therelay (forward faults) and faults occurring behind the relay (re-verse faults). In this paper, the relay direction is arbitrarily optedas the direction of forward faults.In this condition, Fig. 5(a) can be considered for forward

faults. The impedance seen by relay ( ) is expressed as

(19)where and

. Also, is the fault resistance,and and are the impedances between the sources andand the fault point, respectively. These impedances are mainlyinductive and have almost the same angles ( ). Therefore, wehave

(20)

Substituting (20) in (19) yields

(21)

For external faults, considering Fig. 5(b), is equal to

(22)where and

. Hence, it is concluded that

(23)

Now, (22) can be written as

(24)

Comparing (21) and (24), it is clear that the fault resistanceincreases the imaginary part of in internal faults, and de-creases it in external faults. Referring to (15)–(18), the varia-tion of directly impacts the computed mutual impedance( ). Moreover, it can be concluded from (21) and (24) that thepresence of fault resistance increases the real part of and,consequently, the real part of .Modification of relay performance in the presence of fault

resistance can be achieved by rotating the relay characteristic.This process is shown in Fig. 5(c) and (d) where, for internalfaults, the relay characteristic is rotated counterclockwise by apredefined rotation angle (RA), and for external faults, the relaycharacteristic is rotated clockwise by the angle RA. This ap-proach implies that the relay should detect the fault direc-tion before the proper modification of the relay characteristic isperformed. The angle RA is set considering these parameters:line impedance, source impedances, phase difference betweensources, and the maximum value of fault resistance.

B. Power-Flow Direction is Reverse

This situation is shown in Fig. 6(a) for internal faults.can be computed using (19). However, and

will change to and, respectively. There-

fore, the following equations are established:

(25)

(26)

For external faults, considering Fig. 6(b), is stillachieved by (22), where

and . So,it is concluded that

(27)

(28)


Fig. 6. Effects of faults resistance in the reverse power flows. (a) Typical linewith an internal fault. (b) Typical line with an external fault. (c) Relay charac-teristic in internal faults. (d) Relay characteristic in external faults.

Equations (26) and (28) imply that when the power-flow di-rection is reverse, the fault resistance decreases the imaginarypart of in internal faults and increases it in external faults.As shown in Fig. 6(c) and (d), this situation has no undesirableeffect on the relay performance, and it is not necessary to rotatethe relay characteristic.

V. PROPOSED ALGORITHM

The proposed scheme has some features which should be con-sidered in the design of its protective scheme. These features areillustrated as follows.• In normal conditions (when no fault occurs), the powersystem operates as a symmetrical three-phase system. Itmeans that during normal conditions, the power systemis only modeled by a positive-sequence network and itscorresponding zero- and negative-sequence networks areopen circuit. Consequently, the measurement of zero-se-quence impedance and, according to (17) and (18), themutual impedance is not possible during normal condi-tions, and their probable computed values are meaningless.Therefore, a solution should be provided so that the mutualimpedance to be computed is only during faults. For thispurpose, conventional fault detection methods, such as theapplication of superimposed fault currents, can be applied.

• The mentioned problem, that is, the problem of measuringmutual impedance during normal conditions, is indeedpresent in all situations where the power system operatesin balanced conditions. Hence, in the case of three-phasefaults, the proposed relay cannot be used, and other pro-tective schemes should be applied.

Fig. 7. Flowchart of the proposed protective scheme.

• In the case of phase-to-phase faults, the zero-sequence net-work will not be available. Therefore, the proposed relaycannot be applied for phase-to-phase faults.

• Except for three-phase and phase-to-phase faults, theproposed relay is able to protect the transmission lineagainst other types of faults, that is, single-phase and


Fig. 8. Simulated system.

Fig. 9. Internal SLG fault. (a) Three-phase voltages. (b) Three-phase currents.

double-phase-to-ground faults. It should be noted thatsince the short-circuit faults are most probably singlephase, the proposed scheme is applicable in commonpractical cases. However, for solving the shortcoming ofthe proposed scheme in three-phase and phase-to-phasefaults, it is recommended to apply the proposed schemejust as a backup protection.

• As shown in Fig. 3(a), the proposed relay ( ) and theseries compensator are located at the bus . For the faultsoccurring behind the relay , since the fault path seenby relay includes the series compensator, and,therefore, may be computed incorrectly. For solvingthis problem, should be able to detect the fault directionin series-compensated lines. This ability can be providedusing memory voltage polarization. Therefore, the mutualimpedance is computed, only, if detects a forward fault.In this condition, the fault path seen by does not includethe series capacitor and is measured correctly.

Considering these points, the flowchart of the proposed algo-rithm is depicted in Fig. 7. It should be noted that this algorithmdescribes the first zone of the relay, and it can be easily devel-oped for zones 2 and 3.

VI. SIMULATION RESULTS

In this section, the proposed method is tested in differentcases where the series capacitor is placed at the beginning,middle, and both ends of the transmission line.

A. Series Compensation at the Beginning of the Line

Fig. 8 shows the simulated system. The system data are givenin Table III [10]. The line is compensated by a series capac-itor (SC). The degree of compensation is 40%. Moreover, theproposed relay is denoted by .Internal faults are investigated by applying a single-line-to-

ground (SLG) fault on phase at 10% of line . The faultinception instant is 0.5 s, and is zero. Three-phase volt-ages and currents are shown in Fig. 9. Fig. 10 demonstratesperformances of the conventional distance protection and the

Fig. 10. Simulation results for an internal fault. (a) Distance protectionwithout SC. (b) Proposed protection without SC. (c) Distance protection withSC. (d) Proposed protection with SC.

Fig. 11. External SLG fault. (a) Three-phase voltages. (b) Three-phasecurrents.

proposed protection. As shown in Fig. 10(a) and (b), when theline is not compensated, both methods operate correctly and de-tect the internal fault. However, Fig. 10(c) shows that in the se-ries-compensated line, the conventional distance protectionmayfail to operate for close-in faults and the dependability may belost, while Fig. 10(b) and (d) implies that the proposed schemeremains dependable in series-compensated lines.Study on external faults has been carried out by applying an

SLG fault, with , at 10% of line . (It should benoted that we do not worry about the external faults locatedon line , since they are detected as reverse faults by relay.) Fig. 11 shows the measured voltages and currents. Sim-

ilarly, Fig. 12(a) and (b) shows that both protection methodsprovide reliable performances in noncompensated lines. More-over, Fig. 12(b) and (d) demonstrates that the proposed methodis not affected by series compensation. However, as shown inFig. 12(c), the conventional distance protection may experiencean overreach condition for external faults and lose its security.As mentioned in the previous section, the proposed scheme

may be affected by the fault resistance. This issue is investigatedin Fig. 13 which shows that rotating the relay characteristics by


Fig. 12. Simulation results for an external fault. (a) Distance protectionwithout SC. (b) Proposed protection without SC. (c) Distance protection withSC. (d) Proposed protection with SC.

in internal faults and in external faults will increasethe reliability of the proposed scheme.More investigations about the effects of fault resistance and

power-flow conditions are provided in Table I, where the pro-posed protective schemes are denoted by “P.P.” Moreover,is the phase difference between sources and , and is de-fined as . Therefore, the positive means thepower-flow direction is from source to , while the nega-tive is corresponding to power-flow direction from to .Meanwhile, the effect ofMOV is also considered in Table I. Fur-ther, all faults are incepted at 0.7 s, and the computed faultimpedances by P.P. are acquired at 1 s, so that the transientphenomena is vanished.A comparison between the two last columns of Table I shows

that the presence of MOV does not have considerable effects onthe impedance seen by the relay. It should be noted that sincethe characteristic of the proposed relay is inherently reactive,its performance is mainly determined by the imaginary parts ofcomputed impedances (i.e., the computed reactances). An incre-ment of means an increment of line loading would increasethe computed reactances. However, this increment is not con-siderable for the faults located at the end of the protective zone(i.e., 70% of the line). In the worst cases where the line loadingconditions make the relay maloperate, somemodifications, suchas the rotation of relay characteristics, can be performed.Another important parameter which may affect the perfor-

mance of the proposed scheme is the degree of compensation.For this purpose, the proposed scheme is tested with differentcompensation degrees. The results are given in Table II. It canbe considered in this table that the compensation degree mayslightly affect the computed mutual impedance. However, theseeffects are not remarkable.As mentioned in the previous section, the proposed scheme

is applicable in the case of double-line-to-ground (DLG) faults.

Fig. 13. Effects of fault resistance on the proposed scheme. (a) Internal faults.(b) External faults.

TABLE IPERFORMANCE OF THE PROPOSED SCHEME IN DIFFERENT CONDITIONS

TABLE IIEFFECTS OF COMPENSATION DEGREE ON THE PROPOSED SCHEME


Fig. 14. Simulation of the proposed scheme for an ABG fault with .(a) Internal fault without SC. (b) Internal fault with SC. (c) External fault withoutSC. (d) External fault with SC.

Fig. 15. Effects of communication delay on the proposed method.

An example of this issue is shown in Fig. 14, where a DLG faultbetween phases and is applied.According to Section III, an essential requirement of the pro-

posed scheme is the availability of high-speed communicationbetween the local and remote buses of the line. Therefore, itis useful to analyze the effect of communication delay on theproposed scheme. An example of this analysis is provided inFig. 15 for an SLG fault with zero resistance which occurs at

0.7 s. As shown, the communication delay will increase theresponse time of the proposed scheme. However, since the pro-posed scheme is used as backup protection, this delay may beacceptable.

B. Series Compensation at the Middle and Both Ends of theLine

The previous sections imply that the proposed protectivescheme is mainly designed for situations where the seriescompensator is located at the beginning of the transmissionline. In some practical cases, series capacitors may be placed at

Fig. 16. Simulation results where the series capacitors are at the: (a) middleand (b) both ends of line [*Corresponding value of ( ,fault location)].

TABLE IIISIMULATED SYSTEM DATA

the middle, or even at both ends of the line. The performance ofthe proposed method is evaluated in these situations as follows.The system shown in Fig. 8 is changed so that the line

is compensated at the middle point by 40%. In this case, for thefaults located between the relay and the series capacitor (i.e.,the first half of line ), the relay will see the series capac-itor and, therefore, the value of , which is used for the com-putation of , will be affected by the impedance of the seriescapacitor. In other words, due to the negative reactance of theseries capacitor, the imaginary part of will decrease and thelocus of in the - plane will shift down. In Fig. 16(a),the values of are shown for some SLG faults, with dif-ferent locations and various fault resistances. The inside area ofthe polygon depicted in Fig. 16(a) includes the locus of all faults


with 0 to 50 which may located within 80% of the line(In this figure, (0,1) denotes the locus of an SLG fault at 1%

of line with , and so on). As shown in this figure,the operation of the proposed relay is not affected by the loca-tion of the series capacitor, either for internal or external faults.Meanwhile, the relay characteristic can be rotated by tocover the faults with high resistance.The study on the effects of series compensation at both ends

of the line is shown in Fig. 16(b). In this situation, the line ofFig. 8 is compensated by two series capacitors located at bothends of the line. Each series capacitor provides 35% compen-sation degree. In this case, the value of will always be af-fected by the series capacitor. However, the proposed methodwill become reliable since the locus of does not exit fromthe operation region of the relay.

VII. CONCLUSION

In this paper, a new backup distance protective scheme is pro-posed for series-compensated transmission lines. The proposedscheme uses the mutual impedance between phases, instead ofthe positive-sequence impedance, for protection against single-phase and double-phase-to-ground faults. Therefore, it may beused as backup protection. Various simulation studies are pro-vided to evaluate the performance of the proposed scheme. Theobtained results show that the proposed scheme has excellentperformance in comparison with the conventional distance pro-tection in series-compensated transmission lines.

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[19] S. M. Hashemi, M. T. Hagh, and H. Seyedi, “Transmission-line pro-tection: A directional comparison scheme using the average of super-imposed components,” IEEE Trans. Power Del., vol. 28, no. 2, pp.955–964, Apr. 2013.

S. M. Hashemi received the B.Sc. degree in elec-trical engineering from the Bu-Ali Sina University,Hamedan, Iran, in 2007 and the M.Sc. degree inelectrical engineering from the University of Tabriz,Tabriz, Iran, in 2013.His research interests include power system

protection, flexible ac transmission systems, HVDC,power system operation, and electricity markets.

M. Tarafdar Hagh (S’98–M’06) received the M.Sc.(Hons.) and Ph.D. degrees in power engineering fromthe University of Tabriz, Tabriz, Iran, in 1992 and2000, respectively.He has been with the Faculty of Electrical and

Computer Engineering, University of Tabriz, since2000, where he is currently a Professor. He haspublished more than 150 papers in power systemand power-electronics-related topics. His interesttopics include power system operation, flexible actransmission systems, and power quality.

H. Seyedi was born in Iran in 1979. He received theB.Sc., M.Sc., and Ph.D. degrees in electrical engi-neering from the University of Tehran, Tehran, Iran,in 2001, 2003, and 2008, respectively,Dr. Seyedi is currently with the faculty of Elec-

trical and Computer Engineering, University ofTabriz, Tabriz, Iran. His areas of interest includedigital protection of power systems and powersystem transients.

A Novel Backup Distance Protection Scheme for Series-Compensated Transmission Lines

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