A Novel Backup Distance Protection Scheme for Series ... - IEEE Xplore

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

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A Novel Backup Distance Protection Scheme for Series-Compensated Transmission Lines S. M. Hashemi, M. Tarafdar Hagh, Member, IEEE, and H. Seyedi

Abstract—Conventional distance protection applies the positive-sequence impedance to protect a line against short-circuit faults. Series-compensated lines may considerably change the positive-sequence impedance of the fault path and cause the distance relays to maloperate. To overcome this problem, the mutual impedance between phases of a transmission line is used for the design of a new distance protection scheme. The voltages and currents of both ends of the line are applied to compute the mutual impedance between the relay and the fault point. The proposed scheme has a reliable performance in protection against single-phase and double-phase-to-ground faults and, therefore, it can be used as a backup protection. Simulation results approve the efficiency of the proposed method in protection of series-compensated transmission lines. Index Terms—Distance protection, mutual impedance, series compensation, transmission line.

I. INTRODUCTION

S

ERIES compensation is a common practice to increase the loadability of transmission lines and improve the power system stability. Series capacitors are used to compensate a portion of inductive impedance of the line and provide a better voltage profile along the transmission line [1]. Distance relays provide the main protection of transmission lines, as well as interconnected distribution networks [2]. These relays measure the positive-sequence impedance to the fault and compare it with their predefined characteristic. The presence of series capacitor in the fault loop affects reach and directionality of distance relays. The reach of distance relays may experience two major problems: 1) the reduction of series inductance of line which shifts down the locus of fault impedance in the R-X plane, and decreases the reliability of relay and 2) subsynchronous resonance which may introduce remarkable delays in the response of digital phasor estimation methods [3]. Directionality of the distance relay in series-compensated lines may fail due to voltage or current inversions. A common solution for this problem is using memory voltage as the reference quantity to detect fault direction [2].

Manuscript received February 20, 2013; revised May 31, 2013; accepted July 02, 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: m_hashemi89@ms. tabrizu.ac.ir; [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-compensated lines has been an interesting issue for protection engineers and has been the subject of many works for several decades. Effects of series compensation on the performance of directional comparison 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-compensated lines. In [6], the problems of distance protection in series-compensated lines are explained in detail. Series capacitors are commonly protected against transient overvoltages by metal–oxide varistors (MOVs). The nonlinear function of MOV is considered in [7] and [8] to modify the distance protection by computing the voltage drop along the series compensator. This voltage drop is also used in [9], as well as some additional logics, to increase the accuracy and speed of the first zone of distance relays. In [10], some modification techniques are provided for pilot protection schemes to prevent malfunctions of the main line and its adjacent line relays. Problems associated with the directionality of protective relays in series-compensated lines are addressed in [11] where a new process of directional relaying using the phase difference between the prefault and postfault currents is presented. In [12], the residual current is applied to compensate for the impedance measurement error in MHO relays. Proposed protective schemes for series-compensated line are not limited to the distance protection. Differential protection, in the form of the phase comparison scheme, is expected to operate properly in series-compensated lines [13]. Some papers propose protection schemes using the transient features of currents and voltages during fault conditions. These schemes can be mainly divided into the traveling wave-based [14]–[17] and superimposed-based methods [18], [19]. Despite high-speed performance, these schemes are dealing with two challenges: 1) If, for any reason such as measurement errors, the current and voltage samples of the transient time after fault inception are not properly achieved, the schemes cannot operate correctly. 2) In the case of slowly evolving faults, superimposed voltages and current may graduate so slowly that the required traveling waves are not generated. This paper proposes a new distance protection, based on the mutual impedance between phases. Applying the voltages and currents of both ends of the line, the proposed scheme can protect the line against single-phase and double-phase-to-ground faults. Since the mutual impedance is not affected by series compensation, the proposed scheme can provide a reliable backup protection for series-compensated transmission lines. The rest of this paper is organized as follows: Section II analyzes the effects of series compensation on the mutual impedance between

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Fig. 1. Simple transmission line.

Fig. 2. Series-compensated transmission line.

phases. Section III explains that how the mutual impedance is computed in the proposed scheme. In Section IV, the characteristic of the proposed relay is introduced. The proposed protective algorithm and simulation results are presented in Sections V and VI, respectively. Finally, Section VII contains the conclusion of this paper.

Equation (4) implies that the zero-sequence and positive-sequence impedances are given by and , respectively. So the mutual impedance between phases is

(6) II. EFFECTS

SERIES COMPENSATION MUTUAL IMPEDANCE

OF

ON THE

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

Now, suppose that a series compensator with the impedance is located at the beginning of the transmission line (Fig. 2). Applying relations similar to (1)–(4) yields

(1)

(7)

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

(8)

(2)

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

Assuming that all three-phase conductors are the same and the line is fully transposed, it can be stated that and . Therefore, (2) is rewritten as (3) The sequence impedance matrix of and is expressed as

is denoted by

(9) (10) It is obvious from (10) that the presence of the series compensator changes the value of the positive-sequence impedance. Therefore, this impedance is not competent for relaying. However, the mutual impedance does not differ from its value in the noncompensated lines and yet it can be computed by (6). III. COMPUTATION OF MUTUAL IMPEDANCE DISTANCE RELAYING

(4) are known as zero-, positive-, and negawhere , , and tive-sequence impedances, respectively, and is equal to (5)

FOR

In the proposed method, the mutual impedance is computed using (6). However, before using this equation, the zero-sequence and positive-sequence impedances should be calculated. For this purpose, a series-compensated transmission line is considered, as shown in Fig. 3(a). In this figure, the proposed relay is located at the bus , and protects the line . Meanwhile, it is assumed that the voltage and current phasors measured in the remote bus (bus ) are available in the local bus (bus ) using synchronous measurements and high-speed communication between the two buses. For a single-phase-to-ground fault in the phase , the sequence circuits are depicted in Fig. 3(b).

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

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

Using this figure, the following equations can be derived. It should be noted that in the following equations, the subscripts 0, 1, and 2 stand for zero-, positive-, and negative-sequence components, respectively

(11) (12) In (12), is the positive-sequence impedance to fault which is measured by the relay and is given by (13) where

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

, where

(14) Hence, we have (15) Similarly, it can be concluded form (12) that (16)

The computed mutual impedance, similar to the positive-sequence impedance, is proportional to the distance between the relay and fault point. Therefore, the concept of the protective zone in conventional distance protection can be applied to the proposed protective scheme. This means that the first zone may cover 85% of the main line, the second zone can cover the main line plus 50% of the next shortest line, and the third zone may contain the main line plus the next longest line plus a 25% margin. Since the nature of mutual impedance between phases is mainly inductive, a characteristic similar to the reactance relays can be used for the proposed relay. A typical characteristic is represented in Fig. 4. This characteristic is only for the first zone, where is equal to 85% of the mutual reactance of the main protected line. Characteristics of the second and third zone 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 instantaneously, and 10 to 15% of the line end will be protected in zone 2 with a definite time delay. This time delay is unavoidable due to some errors in instrument transformers and inaccuracies in the value of line parameters. For providing an instantaneous protection for the entire line, communication between the local and remote relays can be established to apply a pilot protection scheme, 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 protection. However, this equation neglects the fault resistance. The presence of fault resistance may considerably change the value of (13) and, therefore, makes the computed mutual impedance deviate from its correct value. For analyzing the effect of fault resistance on the performance of the proposed relay, two conditions can be considered, as follows.

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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)

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 of the relay characteristic in external faults.

A. Power-Flow Direction is Forward Before explaining this condition, it should be noted that protective relays of transmission lines are directional. It means that they can discriminate between faults occurring in front of the relay (forward faults) and faults occurring behind the relay (reverse faults). In this paper, the relay direction is arbitrarily opted as 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) and . Also, is the fault resistance, and and are the impedances between the sources and and the fault point, respectively. These impedances are mainly inductive and have almost the same angles ( ). Therefore, we have

Comparing (21) and (24), it is clear that the fault resistance increases the imaginary part of in internal faults, and decreases it in external faults. Referring to (15)–(18), the variadirectly impacts the computed mutual impedance tion of ( ). Moreover, it can be concluded from (21) and (24) that the and, presence of fault resistance increases the real part of . 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 internal faults, the relay characteristic is rotated counterclockwise by a predefined rotation angle (RA), and for external faults, the relay characteristic is rotated clockwise by the angle RA. This apshould detect the fault direcproach implies that the relay tion before the proper modification of the relay characteristic is performed. The angle RA is set considering these parameters: line impedance, source impedances, phase difference between sources, 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. Therefore, the following equations are established: (25)

where

(20) Substituting (20) in (19) yields

(26) For external faults, considering Fig. 6(b), achieved by (22), where and it is concluded that

is still . So,

(27)

(21)

(28)


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

Equations (26) and (28) imply that when the power-flow direction is reverse, the fault resistance decreases the imaginary in internal faults and increases it in external faults. part of As shown in Fig. 6(c) and (d), this situation has no undesirable effect on the relay performance, and it is not necessary to rotate the relay characteristic. V. PROPOSED ALGORITHM The proposed scheme has some features which should be considered in the design of its protective scheme. These features are illustrated as follows. • In normal conditions (when no fault occurs), the power system operates as a symmetrical three-phase system. It means that during normal conditions, the power system is only modeled by a positive-sequence network and its corresponding zero- and negative-sequence networks are open circuit. Consequently, the measurement of zero-sequence impedance and, according to (17) and (18), the mutual impedance is not possible during normal conditions, and their probable computed values are meaningless. Therefore, a solution should be provided so that the mutual impedance to be computed is only during faults. For this purpose, conventional fault detection methods, such as the application of superimposed fault currents, can be applied. • The mentioned problem, that is, the problem of measuring mutual impedance during normal conditions, is indeed present in all situations where the power system operates in balanced conditions. Hence, in the case of three-phase faults, the proposed relay cannot be used, and other protective schemes should be applied.

Fig. 7. Flowchart of the proposed protective scheme.

• In the case of phase-to-phase faults, the zero-sequence network will not be available. Therefore, the proposed relay cannot be applied for phase-to-phase faults. • Except for three-phase and phase-to-phase faults, the proposed relay is able to protect the transmission line against other types of faults, that is, single-phase and

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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 that since the short-circuit faults are most probably single phase, the proposed scheme is applicable in common practical cases. However, for solving the shortcoming of the proposed scheme in three-phase and phase-to-phase faults, it is recommended to apply the proposed scheme just as a backup protection. • As shown in Fig. 3(a), the proposed relay ( ) and the series compensator are located at the bus . For the faults occurring behind the relay , since the fault path seen by relay includes the series compensator, and, therefore, may be computed incorrectly. For solving this problem, should be able to detect the fault direction in series-compensated lines. This ability can be provided using memory voltage polarization. Therefore, the mutual impedance is computed, only, if detects a forward fault. In this condition, the fault path seen by does not include the series capacitor and is measured correctly. Considering these points, the flowchart of the proposed algorithm is depicted in Fig. 7. It should be noted that this algorithm describes the first zone of the relay, and it can be easily developed for zones 2 and 3. VI. SIMULATION RESULTS In this section, the proposed method is tested in different cases 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 given in Table III [10]. The line is compensated by a series capacitor (SC). The degree of compensation is 40%. Moreover, the proposed relay is denoted by . Internal faults are investigated by applying a single-line-toground (SLG) fault on phase at 10% of line . The fault inception instant is 0.5 s, and is zero. Three-phase voltages and currents are shown in Fig. 9. Fig. 10 demonstrates performances of the conventional distance protection and the

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

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

proposed protection. As shown in Fig. 10(a) and (b), when the line is not compensated, both methods operate correctly and detect the internal fault. However, Fig. 10(c) shows that in the series-compensated line, the conventional distance protection may fail to operate for close-in faults and the dependability may be lost, while Fig. 10(b) and (d) implies that the proposed scheme remains 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 be noted that we do not worry about the external faults located on line , since they are detected as reverse faults by relay .) Fig. 11 shows the measured voltages and currents. Similarly, Fig. 12(a) and (b) shows that both protection methods provide reliable performances in noncompensated lines. Moreover, Fig. 12(b) and (d) demonstrates that the proposed method is not affected by series compensation. However, as shown in Fig. 12(c), the conventional distance protection may experience an 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 investigated in Fig. 13 which shows that rotating the relay characteristics by


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

TABLE I PERFORMANCE OF THE PROPOSED SCHEME IN DIFFERENT CONDITIONS

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

in internal faults and in external faults will increase the reliability of the proposed scheme. More investigations about the effects of fault resistance and power-flow conditions are provided in Table I, where the proposed protective schemes are denoted by “P.P.” Moreover, is the phase difference between sources and , and is defined as . Therefore, the positive means the power-flow direction is from source to , while the negative is corresponding to power-flow direction from to . Meanwhile, the effect of MOV is also considered in Table I. Further, all faults are incepted at 0.7 s, and the computed fault impedances by P.P. are acquired at 1 s, so that the transient phenomena is vanished. A comparison between the two last columns of Table I shows that the presence of MOV does not have considerable effects on the impedance seen by the relay. It should be noted that since the characteristic of the proposed relay is inherently reactive, its performance is mainly determined by the imaginary parts of computed impedances (i.e., the computed reactances). An increment of means an increment of line loading would increase the computed reactances. However, this increment is not considerable for the faults located at the end of the protective zone (i.e., 70% of the line). In the worst cases where the line loading conditions make the relay maloperate, some modifications, such as the rotation of relay characteristics, can be performed. Another important parameter which may affect the performance of the proposed scheme is the degree of compensation. For this purpose, the proposed scheme is tested with different compensation degrees. The results are given in Table II. It can be considered in this table that the compensation degree may slightly affect the computed mutual impedance. However, these effects are not remarkable. As mentioned in the previous section, the proposed scheme is applicable in the case of double-line-to-ground (DLG) faults.

TABLE II EFFECTS OF COMPENSATION DEGREE ON THE PROPOSED SCHEME

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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 without SC. (d) External fault with SC.

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

TABLE III SIMULATED SYSTEM DATA

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

An example of this issue is shown in Fig. 14, where a DLG fault between phases and is applied. According to Section III, an essential requirement of the proposed scheme is the availability of high-speed communication between the local and remote buses of the line. Therefore, it is useful to analyze the effect of communication delay on the proposed scheme. An example of this analysis is provided in Fig. 15 for an SLG fault with zero resistance which occurs at 0.7 s. As shown, the communication delay will increase the response time of the proposed scheme. However, since the proposed scheme is used as backup protection, this delay may be acceptable. B. Series Compensation at the Middle and Both Ends of the Line The previous sections imply that the proposed protective scheme is mainly designed for situations where the series compensator is located at the beginning of the transmission line. In some practical cases, series capacitors may be placed at

the middle, or even at both ends of the line. The performance of the 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 the faults located between the relay and the series capacitor (i.e., the first half of line ), the relay will see the series capacitor and, therefore, the value of , which is used for the computation of , will be affected by the impedance of the series capacitor. In other words, due to the negative reactance of the series capacitor, the imaginary part of will decrease and the locus of in the - plane will shift down. In Fig. 16(a), the values of are shown for some SLG faults, with different locations and various fault resistances. The inside area of the 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 location of the series capacitor, either for internal or external faults. Meanwhile, the relay characteristic can be rotated by to cover 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 of Fig. 8 is compensated by two series capacitors located at both ends of the line. Each series capacitor provides 35% compensation degree. In this case, the value of will always be affected by the series capacitor. However, the proposed method will become reliable since the locus of does not exit from the operation region of the relay. VII. CONCLUSION In this paper, a new backup distance protective scheme is proposed for series-compensated transmission lines. The proposed scheme uses the mutual impedance between phases, instead of the positive-sequence impedance, for protection against singlephase and double-phase-to-ground faults. Therefore, it may be used as backup protection. Various simulation studies are provided to evaluate the performance of the proposed scheme. The obtained results show that the proposed scheme has excellent performance in comparison with the conventional distance protection in series-compensated transmission lines.

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[11] P. Jena and A. K. Pradhan, “A positive-sequence directional relaying algorithm for series-compensated line,” IEEE Trans. Power Del., vol. 24, no. 4, pp. 2288–2298, Oct. 2010. [12] A. B. Shah, V. K. Sood, and O. Saad, “Mho relay for protection of series compensated transmission lines,” presented at the Int. Conf. Power Syst. Transients, Toronto, ON, Canada, Sep. 2009. [13] B. Kasztenny, I. Voloh, and E. A. Udren, “Rebirth of the phase comparison line protection principle,” in Proc. 59th Annu. Conf. Protect. Relay Eng., 2006, pp. 193–252. [14] D. W. P. Thomas and C. Christopoulos, “Ultra-high speed protection of series compensated lines,” IEEE Trans. Power Del., vol. 7, no. 1, pp. 139–145, Jan. 1992. [15] J. A. S. B. Jayasinghe, R. K. Aggarwal, A. T. Johns, and Z. Q. Bo, “A novel non-unit protection for series compensated EHV transmission lines based on fault generated high frequency voltage signals,” IEEE Trans. Power Del., vol. 13, no. 2, pp. 405–413, Apr. 1998. [16] A. Y. Abdelaziz, A. M. Ibrahim, M. M. Mansour, and H. E. Talaat, “Modern approaches for protection of series compensated transmission lines,” Elect. Power Syst. Res., vol. 75, pp. 85–98, 2005. [17] C. Aguilera, E. Orduna, and G. Ratta, “Fault detection, classification and faulted phase selection approach based on high-frequency voltage signals applied to a series-compensated line,” Proc. Inst. Elect. Eng., Gen. Transm. Distrib., vol. 153, no. 4, pp. 469–475, Jul. 2006. [18] P. Jafarian and M. Sanaye-Pasand, “High-speed superimposed-based protection of series-compensated transmission lines,” Inst. Eng. Technol. Gen. Transm. Distrib., vol. 5, no. 12, pp. 1290–1300, 2011. [19] S. M. Hashemi, M. T. Hagh, and H. Seyedi, “Transmission-line protection: A directional comparison scheme using the average of superimposed components,” IEEE Trans. Power Del., vol. 28, no. 2, pp. 955–964, Apr. 2013. S. M. Hashemi received the B.Sc. degree in electrical engineering from the Bu-Ali Sina University, Hamedan, Iran, in 2007 and the M.Sc. degree in electrical 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.

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M. Tarafdar Hagh (S’98–M’06) received the M.Sc. (Hons.) and Ph.D. degrees in power engineering from the University of Tabriz, Tabriz, Iran, in 1992 and 2000, respectively. He has been with the Faculty of Electrical and Computer Engineering, University of Tabriz, since 2000, where he is currently a Professor. He has published more than 150 papers in power system and power-electronics-related topics. His interest topics include power system operation, flexible ac transmission systems, and power quality.

H. Seyedi was born in Iran in 1979. He received the B.Sc., M.Sc., and Ph.D. degrees in electrical engineering from the University of Tehran, Tehran, Iran, in 2001, 2003, and 2008, respectively, Dr. Seyedi is currently with the faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran. His areas of interest include digital protection of power systems and power system transients.