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A Novel Distance Protection Algorithm for the Phase-Ground Fault Ying Zhong, Xiaoning Kang, Member, IEEE, Zaibin Jiao, Member, IEEE, Zengchao Wang, and Jiale Suonan, Member, IEEE
Abstract—Traditional distance relays often mis-operate for high transient resistance in phase-ground faults by measuring apparent impedance. To solve this problem, a novel distance protection algorithm is proposed in this paper. It is deduced in the RL lumped transmission line model and calculates the fault distance from the measuring point to the fault point by solving a linear differential equation. In addition, a protection criteria based on the comparison result between the calculated fault distance and the protective setting margin rather than the protective zone measured by apparent impedance is set up in this paper. As a result, the proposed method can avoid overreach for the given characteristics that the calculated fault distance is greater than the actual fault distance when the fault occurs next to the opposite terminal. Meanwhile, both the fundamental component and one decaying DC component are needed to solve the differential equation for the proposed distance algorithm, therefore it can trip the breaker fast during the fault transient period. Finally, EMTP simulation results verify the validity of the proposed distance algorithm on RL transmission line model. However, to put the proposed method into field use, the problem caused by the distributed capacitance of the transmission line on the proposed method will be further studied and solved. Index Terms—Calculated fault distance, distance protection algorithm, linear differential equation, lumped transmission-line model, overreach.
I. INTRODUCTION
D
ISTANCE relays are designed to only operate for faults occurring between the measuring point and the selected reach point while remaining inoperative for all faults outside this region or zone [1]. These days, microprocessor-based distance relays are widely used as the main protection in extremely high voltage/ultra-high voltage (EHV/UHV) transmission lines for their computation ability. However, the basic principle of the distance relay is still the same as the conventional distance relays, namely, measuring the apparent impedance according to the ratio of voltage to current on the assumption that the fault Manuscript received May 01, 2013; revised August 17, 2013; accepted October 04, 2013. Date of publication November 08, 2013; date of current version July 21, 2014. This work was supported in part by the Key Program from National Natural Science Foundation of Chinaunder Grant 51037005) and in part by the National Natural Science Foundation of China under Grant 51177127. Paper no. TPWRD-00521-2013. Y. Zhong, X. Kang, Z. Jiao, and J. Suonan are with the School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China (e-mail:
[email protected]). Z. Wang is with the Electric Power Dispatching and Control Center of Guangdong Power Grid, Guangzhou 510000, China. 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.2013.2286627
point voltage is zero [2]–[4]. The principle usually causes the relays to underreach or overreach when high transient resistance exists in the fault loop circuit. Nevertheless, because of the existence of fault arc and earth resistance, the phase-ground fault in transmission lines usually coincides with high transient resistance, which covers almost 90% of all transmission-line faults in China. The misoperation of distance relays definitely leads to massive losses of energy service. Consequently, the performance of distance relays for phase-ground faults needs to be urgently improved. In order to minimize the influence of transient resistance on conventional distance relays, a lot of research has gone into calculating this resistance. Reference [5] presented a method to compensate transient resistance for distance relays by utilizing the active power at the measuring point. However, the assumption that symmetrical component currents at the measuring point are equal does not always stand when high transient resistance exists as was indicated in this paper. Reference [6] presented a technique of transient resistance compensation in the phase coordinate. The fault impedance was obtained in an iterative manner which improved accuracy. However, the performance of the technique is not validated for multi-infeed transmission lines. Reference [7] presented a method to compensate errors produced by the conventional ground relaying scheme. However, this paper ignored system impedance which will create enormous errors in the scheme for some transmission systems, especially for short-line systems. In order to enhance the tolerance ability of distance relays concerning high transient resistance, a lot of new distance protection principles have been put forward. Assuming the fault component current at the measuring point is in the same phase angle as the fault current in the fault path, a linear equation is proposed to solve the fault distance from the measuring point to the fault point in [8] and [9], respectively. Both methods strengthen distance relays’ tolerance ability of transient resistance. However, when the fault occurs next to the opposite terminal, both relays will overreach. Reference [10] studied a kind of adaptive impedance relay with composite polarizing voltage. However, the accuracy of the estimation of the compensated voltage need not be guaranteed at the time of the fault. In addition, most distance protection principles studied before are based on the fundamental phasors of voltages and currents. Their performance will be deteriorated by the interference of the harmonics in the transient period. Relying on the high performance of optical-fiber sensors [11], [12] put forward an accurate fault-location algorithm by solving a nonlinear equation for the transmission line based on the lumped parameter model. Reference [13] transferred the
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nonlinear equation to a linear equation. They both work out the fault distance using one-terminal data. In this paper, based on the analysis in [12] and [13], a novel distance protection algorithm of calculating the fault distance from the measuring point to the fault point is proposed on the lumped transmission-line model. Assuming the system after the fault point is a pure inductance, the distance protection algorithm is derived in a linear differential equation form. Since the transformers in substations of high-voltage systems are usually grounded directly that lead the opposite system impedance to be mostly reactive, the calculated fault distance by the proposed algorithm is equal to the actual fault distance when the fault occurs at the opposite terminal. Furthermore, when the fault occurs next to the opposite terminal, the assumption will bring positive error into the calculated fault distance by the proposed algorithm. Therefore, the calculated fault distance by the algorithm is greater than the actual fault distance. The theoretical error of the assumption brought into the algorithm is also discussed in this paper. The simulation data from Alternate Transients Program/Electromagnetic Transients Program (ATP/EMTP) verifies the algorithm’s validity to prevent the overreach and the algorithm’s high-tolerance ability of transient resistance. The rest of this paper is organized as follows. Section II analyzes the cause leading to overreach for distance relays and puts forward an assumption that can develop a distance protection algorithm to prevent distance relays from overreaching. Then, the proposed distance protection algorithm is deduced in Section III. In Section IV, the error analysis of the proposed algorithm is discussed and the simulation data from ATP/EMTP to verify the proposed algorithm are described in Section V. II. OVERREACH ANALYSIS OF DISTANCE RELAYS In this section, based on a single-phase system, the cause leading to overreach for distance relays is studied. Afterwards, an assumption that can develop a distance protection algorithm to effectively prevent the overreach for distance relays is deduced. A. Topology of a Single-Phase System A portion of a power system network containing a transmission line between two buses in a single-phase system in the lumped parameter model is shown in Fig. 1. A ground fault, of which the transient resistance value is , occurs at fault point which is away from bus . Suppose that the power flows from terminal to terminal and the relay discussed in this paper is located at terminal . Here, , , and represent the voltage phasors at terminal , terminal , and fault point , respectively. and depict the current phasors from the sending end and the receiving end of the line in fault condition networks, respectively. describes the current phasor in the load condition network. represents the fault current phasor through fault path. and indicate the fault component current phasors from the sending end and the receiving end of the line. , , and display system parameters of resistance, inductance, and impedance at terminal while the system parameter of resistance, inductance, and impedance at terminal are shown by
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Fig. 1. Equivalent circuit of the fault network in a single-phase system. (a) Equivalent circuit of the fault condition network. (b) Equivalent circuit of the load condition network. (c) Equivalent circuit of the fault component network.
, , and , respectively. , , and represent transmission-line resistance, inductance, and impedance per unit length. The total length of the transmission line is denoted by . B. Analysis of the Cause of Overreach for Distance Relays For conventional distance relays, the apparent impedance is defined as the ratio of voltage to current, namely (1) According to the fault loop circuit in Fig. 1(a) (2) Thus, the apparent impedance can be expressed as (3) to terminal , will Since power flows from terminal precede . Therefore, a part of is transferred into capacitive reactance, which will cause overreach for the traditional distance relay. Due to the superposition theory, the current in the fault condition network can be expressed as the sum of the current in the load condition network and the current in the fault component network. Namely (4)
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Therefore, the current through the fault path can be expressed as (5) To eliminate the current variable supplied from the opposite terminal to the transient resistance, according to Fig. 1(c), the relationship between and can be obtained (6) The current distribution factor viewed from the measuring point is defined as
Fig. 2. Locus curve of
along the transmission line.
(7) is the amplitude of , and is the phase angle of . where Substituting (7) into (6), the relationship between and can be expressed as (8) Substituting (8) into (2), the fault-loop circuit equation using only electric quantities at terminal is achieved Fig. 3. Voltage vector diagram when
.
Fig. 4. Voltage vector diagram when
.
(9) In order to overcome the transient resistance’s effect on distance relays, some researchers assume that the current distribution factor viewed from the measuring point is a real number. Therefore (10) is a real number, and represents the calculated fault where distance by (10). In this way, the fault distance can be easily worked out for some distance relays according to (10). According to (7), varies constantly when the fault occurs at the position from the measuring point to the opposite terminal. Meanwhile, in EHV/UHV transmission systems, the system impedance is mostly inductive. Therefore, the phase angle of the system impedance is usually greater than the transmission line’s. Consequently, due to (7), for a certain fault distance , there is 0. Moreover, for a fault distance , there is 0; and for a fault distance , there is 0. Fig. 2 illustrates the locus curve of along a 110-kV transmission system. Definitely, the change of will bring different types of error into by (10) when compared to . When there is due to (9), the voltage vector diagram for a single-phase system is shown in Fig. 3. In Fig. 3, the solid line and according to (9); And the dash line and according to (10). As illustrated in Fig. 2, when , 0. Then, will lag behind by . there is Meanwhile, and are in the same direction, which are both determined by . Therefore, it can be inferred that
. Thus, there is . The distance protection algorithm by (10) can prevent overreach when the fault occurs in the forward direction. Similarly, when there is , the voltage vector diagram is shown in Fig. 4. In Fig. 4, the variables , , , and are defined the same as variables in Fig. 3. Similarly, it can be inferred that in this case. Therefore, the distance protection algorithm in (10) will probably overreach when the fault occurs in the forward direction. C. Introduction of a New Assumption for Distance Relays Based on the analysis in part , that whether the positive error or negative error will be brought into the calculation of is determined by the relative direction of and , namely, the phase angles of and are compared. Moreover, if the phase angle of is greater than that of , positive error will be
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brought into the calculation of the fault distance. Under this condition, the distance relays can effectively avoid the overreach. In high-voltage transmission power systems, transformers in substations that usually ground directly and lead to the opposite system can be regarded as a reactance. Meanwhile, the resistance of the transmission line is quite small compared with the reactance of the transmission line. Once the resistance after the fault point is ignored, an assumption that can develop new distance relays to prevent overreach will come up. In this way, the current distribution factor viewed from the measuring point in the proposed assumption is
where is the resistance before the fault point, and is the total inductance all over the system. Substitute (15) into (14) and multiply the formula by . Then it is achieved that
(11)
(17)
is the fundamental angular frequency, is the inductance after the fault point, is the impedance before the fault point, is the amplitude of , and is the phase angle of . The current distribution factor in (7) is rewritten as
are the function of the In (17), three coefficients fault distance , transient resistance , and the inductance of the opposite terminal , which can be described as
where
(16) Considering , transform (16) by inverse Fourier transformation. The proposed distance protection algorithm of a linear differential equation in time domain is acquired
(12) where point. In order to compare
is the impedance after the fault
(18)
with , (11) and (12) are rewritten as
and are the parameters of the local system, and where they can be easily worked out in real time [12]. B. Modal Transformation Technique for the Three-Phase Transmission System
(13) is always greater than due to (13). It is obvious that According to the analysis above, positive error will always be brought into the calculation of fault distance under the usage of . Therefore, a distance protection algorithm based on this assumption will effectively avoid the overreach. III. DEDUCTION OF THE PROPOSED METHOD
Due to the fault analysis theory for three-phase transmission lines, the three-phase transmission system networks can be transformed into three sequence transmission system networks in order to remove the coupling inference by other phase transmission lines during the fault period. Since the distance protection algorithm is proposed in the time domain, the Clark phase-modal transformation matrix is used in this paper (19)
In this section, a distance protection algorithm calculating the fault distance from the measuring point to the fault point is deduced based on the assumption proposed in Section II. For simplicity, the algorithm is deduced in a single-phase transmission system, and the algorithm for the phase-ground fault in a three-phase transmission system is shown directly.
represents the electric quantity, and subscripts “1,” “2,” and “0” are used with a variable to indicate that they are of mode 1, mode 2, and mode 0, respectively. By using the Clark matrix, the relationship of impedance in modal networks and in-phase networks can be easily obtained
A. Deduction of the Algorithm in a Single-Phase Power System According to the fault network in Fig. 1(a), the following is obtained: (14) In this algorithm, the resistance after the fault point is zero is assumed, as described in Section II. Therefore, the current through the transient resistance path can be expressed as (15)
(20) and depict the self and mutual impedance of the where phase transmission system, respectively. C. Deduction of the Algorithm in a Three-Phase Power System For a phase-ground fault in a three-phase system as shown in Fig. 5 (in this paper, the phase-A-to-ground fault is taken as
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Fig. 5. Equivalent circuit of the fault network for a phase-A-to-ground fault. (a) Equivalent circuit of the fault condition network. (b) Equivalent circuit of the 0 modal network.
an example), the 0 modal network is taken as the fault component network, and the Clark transformation matrix is used as the phase-modal transformation matrix. Since the derivation process of the algorithm for the threephase system is similar to the single-phase system, the distance protection algorithm for the phase-A-to-ground fault is shown without deduction as follows:
(21) , . and where are the transmission-line resistance and inductance in mode 1 p.u. Also, and are the transmission-line resistance and inductance in mode 0 p.u., respectively. The unknown variables in the phase-A-to-ground fault are the fault distance , transient resistance , and the real-time equivalent inductance of the opposite system in mode 0. The relationship between the three variables and the three coefficients in (21) is
Fig. 6. Flowchart.
Since three coefficients exist in (21), theoretically, three voltage and current samples are taken and an equation set of three equations as (21) are composed. Then, the three coefficients can be obtained. In order to reduce error, more than three sampling points are used and an overdetermined equation set is composed. Then, the least-square method is used to solve the overdetermined equation set in this paper. D. Protection Criteria For a phase-ground fault in the three-phase transmission system, the fault distance from the measuring point to the fault point can be calculated according to (21). Thus, the protection criteria of the proposed distance protection algorithm can be inferred as follows: (24)
(22) For the phase-A-to-ground fault, the resistance after the fault point is ignored, namely, 0. Therefore, the current distribution factor viewed from terminal in mode 0 for the proposed algorithm is (23) where is the impedance before the fault point in mode 0 and is the inductance after the fault point in mode 0. By solving (21), the three coefficients can be obtained. With the aid of the relationship in (22), the fault distance can be calculated.
depicts the protective margin of In this paper, the transmission line. Meanwhile, in order to avoid maloperation for faults in the reverse direction, a directional element [14] is used in the proposed distance protective relay. E. Flowchart The proposed distance protective relay runs as the flowchart in Fig. 6. The procedure of running the proposed distance relay mainly includes three parts. The first part is that whether a phase-ground fault that occurs in the protective transmission line will be identified. This will be accomplished by the starting unit and fault phase selector in the distance relay.
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Then, once the fault is identified to be a phase-ground fault, the distance protection algorithm equation according to (21) is set based on the output of the fault phase selector. Furthermore, the fault distance is calculated. Eventually, coordinating with the result of the directional element, once the protection criteria satisfies can the relay trip the breaker. Otherwise, the relay does not. IV. THEORETIC ERROR ANALYSIS In this section, the theoretic calculated fault distance error formula of the proposed distance protection algorithm based on the lumped transmission-line model is studied. For simplicity, the theoretic calculated fault distance error formula of (21) on the fundamental component is deduced based on the lumped transmission-line model in no-load condition. According to Fig. 5, the accurate fault-location formula [12] and the proposed distance protection formula are displayed as
Fig. 7. Error curve of a 110-kV transmission system.
TABLE I PARAMETERS OF THE LINE AND SOURCE IMPEDANCES
(25) (26) where
. . is the impedance after the fault point in mode 0. . is the actual fault distance. Here, is the calculated fault distance of the proposed algorithm in (21). , , and are the phasors of phase A voltage, phase A current, and current in mode 0 at terminal , respectively. Here, a function consisting of variables and is defined as (27) where is the fault distance and is the resistance after the fault point in mode 0. is the same definition as (25). With expanding function at point ( , 0) by Taylor series and ignoring the higher order terms, the following is obtained:
(28) are defined the same as (26). where and According to (25) and (26), it is inferred that (29) Substitute (29) into (28); therefore, it is achieved that (30) Solve the differentials in (30) and then the theoretic calculated fault distance error formula of (21) results (31)
where is a coefficient related to the transient resistance . The error curve of the calculated fault distance by the proposed algorithm in a 110-kV transmission system is shown in Fig. 7 according to (31). The simulation parameters are displayed in Table I. Note that and are known variables in (31). In Fig. 7, the vertical coordinates are defined as (32) As illustrated in Fig. 7, the calculated fault distance of the proposed algorithm in (21) is accurate when the fault occurs at the opposite terminal. Moreover, the assumption brings positive error into the calculated fault distance when the fault occurs next to the opposite terminal. This is in accordance with the analysis in Section II. V. SIMULATION RESULTS For verifying the validation of the proposed algorithm, several fault cases are simulated in a 110-kV transmission system using the ATP/EMTP. The diagram of the studied system is illustrated in Fig. 5. The system operates in 50 Hz, and the parameters of the system are displayed in Table I. The voltage and current data at terminal are measured using a sampling rate of 200 samples/cycle. The calculated fault distance by the proposed method for an A-G solid fault at 40 km away from terminal is shown in Fig. 8. It can be observed that the calculated fault distance is quite accurate, and the fault is identified to occur in the protective zone. To study the transient resistance’s effect on the proposed method, the calculated fault distance for an A-G fault with 100 is shown in Fig. 9. It can be seen that the calculated fault
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TABLE II DISTANCE SIMULATION RESULTS
Fig. 8. Locus curve of the calculated fault distance for a solid fault 40 km away.
Fig. 9. Locus curve of the calculated fault distance for a 100away.
fault 40 km
distance is also accurate despite the high transient resistance during the first period time after the fault occurs. However, the calculated fault distance fluctuates with the lapse of time. This is caused by the fact that dc offset components decay fast in high transient resistance conditions and then (21) becomes ill conditioned with the lapse of time. Therefore, the proposed method cannot be used as a backup protection. To better illustrate the performance of the proposed method, some results are shown in Table II. The method proposed in [8] is named Method i, and the method proposed in this paper is named Method ii. As illustrated in Table II, although Method i is accurate when the fault occurs through a small transient resistance, it will definitely overreach when the fault occurs next to the opposite terminal through high transient resistance. Moreover, 18 km due to the simulation parameters according to (7). When there is , the calculated fault distance is greater than the actual fault distance. And when there is , the calculated fault distance is smaller than the actual distance. This is in accordance with the analysis in Section II. However, the calculated fault distance by Method ii is greater than the actual fault distance when the fault occurs next to the opposite terminal. Namely, the proposed algorithm has a characteristic of positive error. This phenomenon will effectively prevent the distance relays from overreaching. Meanwhile, the proposed algorithm is accurate when the fault occurs at the opposite terminal with small transient resistance. However, the proposed method is inaccurate when the fault occurs at the opposite terminal with high transient resistance for lacking sufficient decaying dc components
to solve (21). This will not occur when rich components exist in the transient period. VI. CONCLUSION A novel distance protection algorithm to calculate the fault distance from the measuring point to the fault point by solving a linear differential equation with three coefficients for phaseground faults is presented in this paper. The calculated fault distance by the proposed algorithm is accurate when the fault occurs at the opposite terminal. Furthermore, the proposed algorithm gets a positive error for the calculated fault distance when the fault occurs next to the opposite terminal. Therefore, the proposed algorithm can effectively prevent distance relays from overreaching. Meanwhile, the proposed algorithm has a high tolerance ability of transient resistance compared to conventional distance relays. However, since the proposed algorithm is based on the lumped transmission-line model, a further distance protection algorithm considering the influence of capacitance will be studied especially for long-distance transmission lines, which always coincide with rich components in the transient period when the fault occurs. In addition, some decaying dc components are needed to solve the three coefficients in the algorithm apart from the fundamental component. This is attainable with the wide use of electronic sensors in China’s smart grid. REFERENCES [1] S. H. Horowitz and A. G. Phadke, Power System Relaying, 3rd ed. Baldock, Hertfordshire, U.K.: Research Studies Press, 2008, pp. 23–40.
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[2] B. J. Mann and I. F. Morrison, “Digital calculation of impedance for transmission line protection,” IEEE Trans. Power App. Syst., vol. PAS-90, no. 1, pp. 270–279, Jan. 1971. [3] M. S. Sachdev and M. A. Baribeau, “A new algorithm for digital impedance relays,” IEEE Trans. Power App. Syst., vol. PAS-98, no. 6, pp. 2232–2240, Nov. 1979. [4] D. D’Amore and A. Ferrero, “A simplified algorithm for digital distance protection based on Fourier techniques,” IEEE Trans. Power Del., vol. 4, no. 1, pp. 157–164, Jan. 1989. [5] M. M. Eissa, “Ground distance relay compensation based on fault resistance calculation,” IEEE Trans. Power Del., vol. 21, no. 4, pp. 1830–1835, Oct. 2006. [6] A. D. Filomena, R. H. Salim, M. Resener, and A. S. Bretas, “Ground distance relaying with fault-resistance compensation for unbalanced systems,” IEEE Trans. Power Del., vol. 23, no. 3, pp. 1319–1326, Jul. 2008. [7] V. H. Makwana and B. R. Bhalja, “A new digital distance relaying scheme for compensation of high-resistance faults on transmission line,” IEEE Trans. Power Del., vol. 27, no. 4, pp. 2133–2140, Oct. 2012. [8] Z. Y. Xu, S. J. Jiang, Q. X. Yang, and T. S. Bi, “Ground distance relaying algorithm for high resistance fault,” IET Gen. Transm. Distrib., vol. 4, no. 1, pp. 27–35, Jan. 2010. [9] Z. Y. Xu, G. Xu, R. Li, S. Yu, and Q. X. Yang, “A new fault-impedance algorithm for distance relaying on a transmission line,” IEEE Trans. Power Del., vol. 25, no. 3, pp. 1384–1392, Jul. 2010. [10] Q. K. Liu, S. F. Huang, H. Z. Liu, and W. S. Liu, “Adaptive impedance relay with composite polarizing voltage against fault resistance,” IEEE Trans. Power Del., vol. 23, no. 2, pp. 586–592, Apr. 2008. [11] M. S. Ricardo, M. Hugo, and N. Ivo, “Optical current sensors for high power systems: A review,” Appl. Sci., vol. 2, no. 3, 2012. [12] J. L. Suonan and J. Qi, “An accurate fault location algorithm for transmission line based on R-L model parameter identification,” Elect. Power Syst. Res., vol. 76, no. 1–3, pp. 17–24, 2005. [13] J. L. Suonan, Z. C. Wang, and X. N. Kang, “An accurate fault location algorithm based on parameter identification of linear differential equation using one terminal data,” in Proc. 4th Int. Advanced Power Syst. Autom. Protect. Conf., Oct. 2011, vol. 1, pp. 407–412. [14] J. L. Suonan, X. B. Wang, and X. L. Meng, “Quick directional elemental based on R-L model parameter identification,” (in Chinese) J. Xi’an Jiaotong Univ., vol. 40, no. 6, pp. 68–693, Jun. 2006. Ying Zhong was born in Chongqing, China, on April 1, 1988. He is currently pursuing the M.Sc. degree in electrical engineering from Xi’an Jiaotong University, Xi’an, China. His research interests include transmission-line fault location and protection.
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Xiaoning Kang (M’10) was born in Shaanxi, China, on March 2, 1968. His research interests include fault location, substation automation, and transformer protective relaying.
Zaibin Jiao (M’10) received the B.Sc. and M.Sc. degrees in electrical engineering from Southwest Jiaotong University, Chengdu, China, and the Ph.D. degree in electrical engineering from Xi’an Jiaotong University, Xi’an, China, in 2008. He joined Xi’an Jiaotong University in 2008, and is currently a Lecturer. From 2011 to 2012, he visited the University of Hong Kong as a Postdoctoral Fellow. His areas of interest are power system protection and smart grid.
Zengchao Wang was born in Shandong, China, on June 14, 1987. Currently, he is an Assistant Engineer with the Electric Power Dispatching and Control Center, Guangdong Power Grid, Guangzhou, China. His research interests include transmission-line fault location and protection.
Jiale Suonan (M’10) was born in Xinjiang, China, on August 15, 1960. He has been a Professor of Electrical Engineering with the Department of Electrical Engineering, Xi’an Jiaotong University, Shaanxi, China, since 1996. His research is mainly in extremely high voltage/ultrahigh voltage (EHV/UHV) transmission-line protections. He has developed many devices of transmission-lines protection that are running in power systems in China.
