Transmission-Line Protection - IEEE Xplore

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 28, NO. 2, APRIL 2013

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Transmission-Line Protection: A Directional Comparison Scheme Using the Average of Superimposed Components S. M. Hashemi, M. Tarafdar Hagh, Member, IEEE, and H. Seyedi

Abstract—In this paper, a new protection scheme for transmission lines is presented. The method has some advantages in comparison with conventional line protection schemes. Faster fault detection and instantaneous coverage of almost 100% of the line are the main advantages of the new method. A full-cycle averaging window is used for fault detection. While the power system is in normal operation conditions, this average is approximately equal to zero. As soon as the faulty signals enter the window, the average is changed to a nonzero value. It is shown that the product of this average value for voltage and current of the faulty phase, in a specific time interval after fault inception, is negative for the forward faults and positive for the reverse faults. The fault is detected by communication between the local and the remote relays. Simulation and experimental results show the efficiency of the proposed method in fast detection of line faults in less than a half cycle. Index Terms—Directional comparison, protection, relaying, superimposed component, transmission line.

I. INTRODUCTION

T

RANSMISSION lines are prevalently protected by distance relays as the main protection, and overcurrent relays as the backup protection. Both distance and overcurrent protections use fundamental or power frequency components to detect the faults. In microprocessor relays, the extraction of fundamental frequency voltages and currents is, conventionally, provided by phasor estimation methods such as the Fourier algorithm [1], [2]. The common required time for fault detection in these relays is approximately one to two cycles. Fast detection and clearing of faults improves the stability of power systems, especially in extremely high voltage (EHV) transmission lines. Therefore, the trend is toward faster protection schemes in modern integrated power systems. Superimposed components are changes in voltage and current signals with respect to the normal or steady-state conditions. These changes cause voltage and current traveling waves (TWs) to propagate away from the fault location. Protection schemes using TWs are capable of detecting the fault in the first milliseconds following the fault inception [3], [4]. TW-based protection has some outstanding features, such as immunity to power Manuscript received May 08, 2012; revised September 07, 2012; accepted October 16, 2012. Date of publication February 05, 2013; date of current version March 21, 2013. Paper no. TPWRD-00476-2012. 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.2012.2226609

swings, current-transformer (CT) saturation, and long lines capacitance which make it more robust than traditional distance protection [5]. TW-based protection may determine the fault direction by comparing the polarities of superimposed quantities of voltages and currents. Besides, the protection can measure the fault distance using the time difference between forward and backward TWs. A common feature is observed among many TW-based protection schemes in the literature [6]–[9]. These schemes usually require high sampling frequency between several hundreds of kilohertz to 1 MHz, which is more than the sampling rate of conventional digital relays [7]. Furthermore, limitations in the bandwidth of conventional VTs and CTs, are also introduced and there are some difficulties in measuring superimposed components for TW applications [10], [11]. Superimposed-based protection is not limited to TWs. It is shown in [12]–[14] that distance protection using superimposed components, so-called delta quantities, instead of fundamental frequency impedance, may solve some problems in the conventional distance relaying. Superimposed currents are proposed in [15] for the phase comparison protection to remove the sensitivity of this protection to heavy load conditions. In [16], superimposed components are used for providing high-speed directional comparison bus protection. The transient energy produced by superimposed components is used in [17] for directional comparison relaying. Positive-sequence superimposed components are used in [18] for directional protection of EHV transmission lines. In [19], it has been shown that superimposed-based directional comparison offers some benefits compared with the line differential protection. This paper introduces a directional comparison protective scheme using the average value of superimposed components. The method is able to detect the faulty phase and the fault direction in less than a half-cycle. Applying the sampling rate of 64 samples/cycle makes the proposed method compatible with the commercial relays. Moreover, using the average of voltage and current signals in a full cycle for fault detection, the buffer size applied in the proposed method is reduced compared with some superimposed-based protection schemes where the samples of two or four full cycles are required to be saved [11], [20]. II. BRIEF REVIEW ON THE FAULT SUPERIMPOSED COMPONENTS Consider a simple transmission system, shown in Fig. 1(a). This fault can be modeled by a voltage source, which is equal in magnitude and opposite in sign to the prefault voltage at the fault point [2]. According to the superposition theorem, the

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Fig. 1. Simple transmission system with (a) steady-state (prefault) and (b) superimposed-state representation. A three-phase fault is assumed to occur in the forward direction, with respect to the relay R.

changes in the relay point (R) voltage and current might be computed by zeroing all prefault voltage sources and representing all network components and loads with their impedances [21]. The obtained circuit, which is depicted in Fig. 1(b), is known as a superimposed network. For simplicity, only the inductive portion of impedances is considered in Fig. 1(b). Therefore, the postfault voltage and current at any point of the network can be acquired by superposition of their prefault values and superimposed values, as (1) is the superimposed voltage. A similar relation where may be written for the current. The superimposed components contain dc offset, harmonics, and high-frequency transients. Their salient feature is that the product of superimposed voltage and current at the relay point is negative for the forward faults and positive for the reverse faults. These properties, providing an excellent criterion for directional comparison relaying, form the basis of TW-based protective schemes [2]. In digital protection, the superimposed components may be extracted by subtracting each sample from its corresponding sample in the previous cycle. This process extracts the superimposed components in only one cycle after fault inception, and the relay would take its decision in this interval. III. PROPOSED PROTECTIVE METHOD In the steady-state conditions, the voltage and current signals in the transmission lines are almost pure sinusoidal. This implies that the average value of voltage and current signals, in steady-state conditions, is almost equal to zero. Referring to (1), it may be concluded that the average value of postfault voltage (or current) is equal to that of superimposed voltage (or current)

Fig. 2. Output of the averaging filter for a typical current waveform.

called the averaging filter. The filter proceeds sample by sample along the input signal. While the protected transmission system operates in healthy conditions, the input signal, whether voltage or current, has a pure sinusoidal waveform, and the output of the filter is near zero. As soon as the faulty samples enter the filter, its output changes to a nonzero value. As mentioned before, this value is equal to the average of the superimposed components. Referring to Fig. 1(b), the superimposed components can be considered as the zero-state response of the electric circuit. According to the electrical circuit theory, this response consists of two parts, which are transient and steady-state responses, respectively. For example, if the transmission line is modeled by a series branch, the superimposed current consists of one decaying dc component and one steady-state sinusoidal component. The average of this current is, therefore, equal to its dc value. However, as shown in Fig. 2, since the input signal passes sample by sample through the averaging filter, one cycle should be elapsed after the fault inception instant in order for that output of filter to become equal to the signal dc value. Moreover, the output of the filter during this cycle has an interesting feature which is investigated for forward and reverse faults, as follows. A. Forward Faults Considering Fig. 1, suppose that a three-phase fault occurs in the forward direction, with respect to the relay R, at point F. Shifting the time origin to the fault inception instant, the superimposed voltage and current can be computed as (4)

(2)

(5)

where denotes the average value of the periodic signal , with the period , which is represented in the continuous time form by . In the discrete time form, the average value can be represented as

(6)

(3) where is the number of samples per cycle. Selecting a data window with the length of one cycle, the above equation may be

The discrete time representations of (5) and (6), assuming and can be written as (7) (8)

HASHEMI et al.: TRANSMISSION-LINE PROTECTION

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TABLE I DETERMINING THE SIGN OF

IN THE

FIRST CYCLE AFTER

FAULT INCEPTION

Fig. 3. One-line diagram of a simple transmission system with (a) steady-state (prefault) and (b) superimposed-state representation. A three-phase fault is assumed to occur in the reverse direction, with respect to the relay R.

B. Reverse Faults For the reverse faults condition, Fig. 3 can be investigated

(9) (10)

(11) Here, assuming and , the discrete time representations of (9) and (10) can be written as (12) (13) Comparing (7), (8), (12), and (13) and considering that , and are positive values, it can be realized that the sign of in the forward faults is opposite that sign in the reverse faults, while the sign of is similar in both forward and reverse faults. This property may be used as a criterion for discriminating between the forward and the reverse faults. For this purpose, let us compute the product of and . For the forward faults, we have

(14) For the reverse faults

(15) The sign of in the first cycle after fault inception is given in Table I, where it can be shown that this sign is dependent on the sample number . Moreover, is restricted by the number of samples per cycle and the value of fault inception angle . For particular values of , the interval

where the sign of is invariant, becomes very short and may include only one or two samples. It means that the dependability of the relay would decrease. Since the value of is variable, can be controlled only by . In other words, for increasing the interval where the sign of is negative for forward faults and positive for reverse faults, the sampling frequency should be increased. As before, this increment is a shortcoming for practical implementation in the conventional relays. This problem can be solved by the proposed method, as follows. The average value of the voltage and current signal in each phase is calculated using (3). Assume is the number of superimposed samples entering the averaging window. The output of the window for forward faults is given by (16) and (17) at the bottom of the next page. For the reverse faults, similar equations result by replacing in (16) with , and in (17) with . It can be shown that the first interval after the fault inception instant, where the sign of is invariant, is equal to for and for . Comparing these values with Table I demonstrates that the interval becomes almost twice. In Fig. 4, the product of (16) and (17) is compared with (14) for two different fault inception angles. It is shown that the mentioned interval is extended. The proposed method consists of (16) and (17)of the following stages. The rate of sampling is assumed to be 64 samples/cycle. The relay detects the fault using eight consecutive superimposed samples. 1) Phase Selection: Phase selectivity provides the capability of single-phase automatic reclosing. On overhead lines, most faults are of a transient nature and disappear when the infeed is switched off. Therefore, following the fault clearance, the line can be returned to service [12]. This means that the single-phase tripping is preferred for single-phase-to-ground faults. Singlephase automatic reclosing basically improves the transient stability of power systems. The process of phase selection in the proposed method is performed by comparing the absolute value of the average of superimposed currents in the eight consecutive windows with a threshold value. This value can be selected similar to the setting of overcurrent relays. It means that assuming

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(17) for various fault inception

Fig. 5. Directional relaying by the proposed method in phase A. A similar process is performed in phases B and C.

the minimum fault current level of 1.25 times the maximum load current, the threshold value may be set to , where is the rated current in the secondary side of CT which is typically 5 A. (The division to is because of the averaging nature of the proposed method.) Single-phase tripping is executed by the proposed algorithm if a single-phase fault is detected for the first time. Otherwise, all three phases should be tripped. 2) Detecting the Fault Direction: A flowchart of the proposed directional relaying is shown in Fig. 5. When a short-circuit fault occurs, it changes the output of the averaging filter to a nonzero value. However, because of VT and CT errors and some environmental noises, during normal operation of the system and in the absence of any fault, this output may not be exactly equal to zero. A threshold level is, therefore, needed to detect the fault conditions. This threshold for the current signals is selected as the aforementioned value (i.e., ), and for the voltage signals, is assumed to be , where is the rated voltage in the secondary side of VT which is typically 110 V. Provided that the average of superimposed voltages and currents

exceeds their corresponding thresholds, the relay computes the sum of and the sum of in the eight consecutive windows. The product of these values forms the basis of directional relaying: The negative indicates forward faults, while the positive indicates reverse faults (Fig. 5). In very rare situations where the value of becomes almost zero , the fault direction is detected in one or two subsequent windows. 3) Communication With the Remote Relay: So far, the relay discriminates between forward and reverse faults. Additional discrimination should be performed between the internal faults (i.e., the faults between the close-in and the remote bus and the external faults, that is, the faults beyond the remote bus). This can be provided by communication between local and remote relays. Assume the forward direction for the local and the remote relays as depicted in Fig. 6. If one relay detects a fault that is in the forward direction, it will wait for the permissive signal from the remote bus relay. The internal fault is, therefore, detected if both relays detect the fault in the forward direction. This provides extremely fast protection for the entire line.

Fig. 4. Normalized values of (14) and (16) . (b) . angles. (a)

(16)

(17)


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Fig. 6. Directional comparison relaying by the proposed method.

Fig. 7. First simulated system.

TABLE II SIMULATION RESULTS FOR THE STUDY ON THE EFFECT OF FAULT LOCATION OF THE PROPOSED METHOD

Fig. 8. Simulation result of an Ag fault in the forward direction. (a) Voltage and current of phase A. (b) Averages of the superimposed voltage and current.

IV. SIMULATION RESULTS The proposed method is tested on two different systems under several operating conditions. The results are given in the following subsections A and B. A. System I This system is a part of an Iranian 230-kV transmission network, depicted in Fig. 7. The system data are given in Table X. The system is simulated in PSCAD/EMTDC where the voltage and current signals in the relay point are used for running the proposed scheme in MATLAB. The frequency-dependent model is used for transmission lines in order to increase the accuracy of simulation. Performance of the proposed method is evaluated in many various conditions which are summarized as follows. 1) Effect of Fault Location: Various types of fault in different points of the transmission line are tested. Table II shows that the proposed algorithm is able to detect the fault in less than a half cycle. In this table and the following tables, Ag, AB, ABg, and ABC stand for single-phase-to-ground, doublephase, double-phase-to-ground, and three-phase faults, respectively. Moreover, F and R denote forward and reverse, and the negative locations represent the faults occurring in the reverse direction. For the fault Ag at 10 km far from bus B, the details of waveforms are shown in Fig. 8, where the system frequency is

50 Hz. For the internal faults as , both relays R and R should detect the fault in the forward direction while for the external faults as , relay R, on the contrary of relay R , should detect the fault in the reverse direction. Neglecting the delay of the communication channel, the total time required for fault detection is determined by the relay with longer operating time. 2) Effect of Fault Resistance: The presence of resistance in the fault path causes the dc component of fault current in (7) and (10) (i.e., the term to decay exponentially). Since the fault resistance is remarkable in the case of earth faults, performance of the proposed method is tested on some single-line-to-ground faults with fault resistances between 5 to 100 . As shown in Table III, the proposed method seems to not be sensitive to the fault resistance. 3) Effect of Fault Inception Angle: The presence of the dc component in the fault current depends on the fault inception angle. In other words, it would be some angles, or equally some instants, that the corresponding fault current does not have any dc value. Nevertheless, as the averaging window moves sample by sample, it needs at least one full cycle to compute the new dc value after fault inception. In this interval, the estimated dc component changes from zero to a nonzero value and returns to zero at the end of one cycle. Fig. 9 represents an example of this condition for a reverse three-phase fault at 0.204 s. The effect of the fault inception angle on the proposed method can be considered in Table IV. In order to cover the entire interval of 0 to , the fault inception instant is gradually increased in one full cycle. The results of Table IV demonstrate that the required time for fault detection has increased in some

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TABLE III SIMULATION RESULTS FOR THE STUDY ON EFFECT OF FAULT RESISTANCE ON THE PROPOSED METHOD


TABLE IV SIMULATION RESULTS FOR THE STUDY ON EFFECT OF FAULT INCEPTION ANGLE ON THE PROPOSED METHOD

TABLE V SIMULATION RESULTS IN THE CASE OF NOISY SIGNALS

Fig. 9. Example of zero dc current for a particular fault inception angle. Only phase A is represented.

inception angles. However, the delay, in the worst condition, is less than half a cycle. 4) Effect of Noisy Fault Signals: Since the nature of the proposed method is averaging signals during one full cycle, it is expected that the noise effect on the proposed method is not considerable. This is due to the fact that the average of white noise is zero. Various faults with different signal-to-noise ratios (SNRs) are tested, and the results are shown in Table V. For this purpose, the Gaussian white noise with the signal-to-noise ratio (SNR) of 60, 40, and 20 dB are added to the main signals. For increasing the security of the proposed method in the presence of noise-polluted signals, the value of thresholds should be set more accurately. The inception instant of all faults in Table V is 0.2 s. B. System II The purpose of simulation studies in this case is to evaluate the performance of the proposed method in situations where the line distance protection, as the prevailing protection in transmission lines, is encountered with challenges and difficulties. The IEEE Power System Relaying Committee (PSRC) proposes the system depicted in Fig. 10 for testing most transmissionline protection applications [22]. The system is simulated in

the Electromagnetic Transients Porgram (EMTP). It should be noted that for each of the following cases, according to [22], some changes on the topology of Fig. 10 are applied. The system frequency is 60 Hz, and the sampling rate is 64 samples/cycle. Referring to Table X, the protection in two challenging conditions (i.e., the presence of short line and the maximum line loading) has been considered in the previous subsection. The other cases are investigated as follows.


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TABLE VII SIMULATION RESULTS FOR EVALUATING THE PROPOSED METHOD THREE-TERMINAL LINES

IN

Fig. 10. IEEE PSRC recommended model for testing transmission-line protection in EMTP.

TABLE VI SIMULATION RESULTS

FOR EVALUATING THE PROPOSED DOUBLE-CIRCUIT LINES

METHOD

IN

TABLE VIII SIMULATION RESULTS FOR EVALUATING THE PROPOSED METHOD IN POWER SWING CONDITIONS

1) Double-Circuit Lines: Mutual inductive coupling between transmission lines on the same tower or parallel along the same right of way introduces some errors in the impedance measured by distance relays. These errors are negligible in positive- and negative-sequence impedances. Nevertheless, as the zero-sequence mutual impedance of parallel lines can be 50% to 70% of zero-sequence self-impedance, the impact of mutual coupling is more significant in the case of ground faults [12], [23]. Therefore, the performance of the proposed method in parallel lines is evaluated only for the earth faults. For this purpose, the switch SW in Fig. 10 remains open and the relay is tested for faults on 66% of the lower line AB, and on 11% of the upper line AB. The results are given in Table VI. 2) Three-Terminal Lines: Sometimes, usually due to economic restrictions, transmission lines are tapped to provide intermediate connections to loads, or to reinforce the underlying lower voltage network through a transformer. These connections introduce some problems in distance protection, especially when sources of generation exist behind the tap points [24]. The fundamental problem with this line configuration is the intermediate infeed to the fault location from the third terminal [12]. Besides, the application of travelling-wave-based protection in three-terminal lines requires careful study, since the travelling waves are strongly affected by the connected taps. To create this condition in Fig. 10, the upper line AB is opened and the switch SW is closed. The tap point is located at 33% of the length of line AB. Afterwards, the performance of relays R1 and R2 is evaluated for the faults located at F1

(at 66% of the length of line AB) and F3 (at 33% of the length of line BD). As considered in Table VII, the results show the efficiency of the proposed algorithm in this case. 3) Power Swings: From the reliability point of view, the distance relays have two fundamental problems in the presence of power swings. The first problem is the possibility of detecting power swing as a fault, which causes the loss of security. For preventing these conditions, distance relays are equipped with the power swing blocking (PSB) units, which blind the relay to see the faults while the power swing persists. This function, however, leads to the second problem, which is loss of dependability for the faults occurring during a power swing. The problem is considerable only for symmetrical or three-phase faults [25], since asymmetrical faults can be detected by other protective approaches, like applying the negative-sequence components. The proposed method is, however, immune to the power swing conditions. According to [22], power swing can be created in Fig. 10 by applying a three-phase fault at bus A and removing the fault before the generator loses synchronism. The second cited problem is investigated by applying three-phase faults at points and (defined in Subsection B.2), where the results are presented in Table VIII.

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TABLE IX SIMULATION RESULTS FOR EVALUATING THE PROPOSED METHOD IN SERIES-COMPENSATED LINES

Fig. 11. Experimental prototype setup.

4) Series-Compensated Lines: Compensation of transmission lines by series capacitors imposes some significant effects on both directionality and reach of distance relays. The details are out of the scope of this paper and can be found in [26] and [27]. These effects become more severe in double-circuit series-compensated lines. In Fig. 10, the source -connected line is removed and the double-circuit line AB is compensated by the degrees of 70%. The performance of relays and is tested for the faults at points and , where the results are given in Table IX. With protection of series-compensated lines, the location of the fault has a key role, since the well-known phenomena of voltage and current inversions are affected by the fault location. For the sake of briefness, these phenomena are not investigated further. However, they are considered in Table IX. The presence of metal–oxide varistors (MOVs) is also considered in this table. As shown, the presence of two capacitors in the path of reverse faults at point , in some situations, makes the direction detected by relays and inversed. However, since this inversion occurs for both relays, it does not impact the fault detection criterion used by the proposed method. It should be noted that this condition is only present in double-circuit series-compensated lines. Our extensive studies show that this inversion is not present in single-circuit series-compensated lines. It is remarkable that, in comparison with Fig. 2, the output of the averaging filter in series-compensated lines is equal to that of the zero-state response of the second-order circuit composed by

Fig. 12. Experimental results of an SLG fault on phase A: (a) three-phase voltages, (b) three-phase currents at the relaying point, (c) averages of voltage and current of the faulty phase, (d) output of the proposed method in simulation, (e) experimental averages of voltage and current computed by the relay, and (f) the experimental result of the fault direction by the relay.

the series capacitor and the line series impedance which consists of subsynchronous oscillations. V. EXPERIMENTAL RESULTS The proposed method is tested, also, on an experimental prototype setup, which is depicted in Fig. 11. The relay is designed using an AVR microcontroller (ATMEGA32A). In order to make the laboratory tests compatible with the real world, the real voltage and current signals saved by a fault recorder


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Fig. 12(c). The output of the proposed method, which is provided by simulation, is represented in Fig. 12(d). In this figure, the fault direction signal is 0, where there is no fault or there is a reverse fault. As soon as a forward fault is detected by the relay, this signal becomes 1. The results of the experimental test are shown in Fig. 12(e) and (f), where the relay has detected the fault direction within less than a quarter of a cycle after the fault inception. Another experimental test is carried out for an SLG fault on phase C, where the obtained results are shown in Fig. 13. VI. CONCLUSION This paper presents a high-speed directional comparison protective scheme for transmission lines, using the average value of superimposed components. Extensive simulation studies are performed to evaluate the proposed method in different operating conditions, including double-circuit lines, three terminal lines, power swing conditions, and series-compensated lines. The impact of important parameters, such as fault resistance, fault location, fault inception angle, and noise-polluted fault signals on the protection systems are also considered in evaluating the proposed method. The obtained simulation results, in addition to the experimental results, show that the proposed method is competent for being applied to line protection. Fig. 13. Experimental results of an SLG fault on phase C: (a) three phase voltages, (b) three-phase currents at the relaying point, (c) averages of voltage and current of the faulty phase, (d) output of the proposed method in simulation, (e) experimental averages of voltage and current computed by the relay, (f) and experimental result of the fault direction by the relay.

TABLE X PARAMETERS OF THE SYSTEM I

have been used. These data include three-phase voltages and currents measured at the relaying point in the substation of one side of the line. These voltages and currents are saved into a personal computer and played back as the inputs of the relay. Considering restrictions in the space of this paper, only the results of two single-phase-to-ground (SLG) faults are presented here. All faults are internal and, therefore, the protective relays of the line had to trip the line. Fig. 12 shows the result of an SLG fault on phase A. The voltages and currents of three phases, which are acquired by the fault recorder, are depicted in Fig. 12(a) and (b), respectively. The averages of voltage and current of phase A, which are used for the detection of fault direction, are depicted in

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[16] M. R. D. Zadeh, T. S. Sidhu, and A. Klimek, “Implementation and testing of directional comparison bus protection based on IEC61850 process bus,” IEEE Trans. Power Del., vol. 26, no. 3, pp. 1530–1537, Jul. 2011. [17] J. Tan, D. Tholomier, G. Bin, and W. Hua, “Sensitivity and stability of superimposed component based directional comparison protection,” in Proc. Can. Conf. Elect. Comput. Eng., 2007, pp. 280–283. [18] H. Gao and P. A. Crossley, “Design and evaluation of a directional algorithm for transmission-line protection based on positive sequence fault components,” in Proc. Inst. Elect. Eng., Gen. Transm. Distrib., Nov. 2006, vol. 153, no. 6, pp. 711–718. [19] D. Tholomier, S. Richards, and A. Apostolov, “Which one is better-line differential or directional comparison?,” in Proc. 9th Int. Conf. Develop. Power Syst. Protect. Inst. Eng. Technol., Montreal, QC, Canada, 2008, pp. 86–91. [20] A. P. Apostolov, D. Tholomier, and S. H. Richards, “Superimposed components based sub-cycle protection of transmission lines,” in Proc. IEEE Power Eng. Soc. Power Syst. Conf. Expo., New York, 2004, pp. 592–597. [21] H. Saadat, Power System Analysis. New York: McGraw-Hill, 1999. [22] Power Syst. Relaying Comm., “EMTP reference models for transmission line relay testing,” Final, Tech. Rep., 2005. [Online]. Available: www.pes-psrc.org [23] H. Seyedi and L. Behroozi, “New distance relay compensation algorithm for double-circuit transmission line protection,” Inst. Eng. Technol. Gen. Transm. Distrib., vol. 5, no. 1, pp. 1011–1018, 2011. [24] S. H. Horowitz and A. G. Phadke, Power System Relaying, 3rd ed. Hoboken, NJ: Wiley, 2008. [25] C. Pang and M. Kezunovic, “Fast distance relay scheme for detecting symmetrical fault during power swing,” IEEE Trans. Power Del., vol. 25, no. 4, pp. 2205–2212, Oct. 2010. [26] P. M. Anderson, Power System Protection. New York: IEEE, 1999. [27] 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.

S. M. Hashemi received the B.Sc. degree in electrical engineering from the Bu-Ali Sina University, Hamedan, Iran, and is currently pursuing the M.Sc. degree in electrical engineering at the University of Tabriz, Tabriz, Iran. 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 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, . Currently, he is 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