Overcurrent Relays Coordination in Interconnected ... - IEEE Xplore

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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 30, NO. 2, APRIL 2015

Overcurrent Relays Coordination in Interconnected Networks Using Accurate Analytical Method and Based on Determination of Fault Critical Point Davood Solati Alkaran, Mohammad Reza Vatani, Mohammad Javad Sanjari, Gevork B. Gharehpetian, and Abdul Halim Yatim, Senior Member, IEEE

Abstract—Overcurrent relays should be coordinated for different operation conditions in interconnected networks. In addition, the time interval should not reach under the predefined threshold for each pair of primary and backup relays for the faults occurring in the protection zone of the primary relay. An analytical approach is presented in this paper to calculate the impedance matrix of the network in fault condition in order to determine the critical fault point accurately. Overcurrent relays are coordinated by using the presented critical fault point instead of the close-in fault used in previous studies. Simulation results show the accuracy of the proposed method for overcurrent relays coordination in comparison with other methods.

Operation time of the th relay for the fault located in front of it. Operation time of the backup relay for the fault at the critical fault point (in seconds). Operation time of the primary relay for the fault at the critical fault point (in seconds). Impedance matrix of the network. Total short circuit current (in amperes). Prefault voltage at bus (in volts or per unit).

Index Terms—Coordination time interval, critical fault point, optimal coordination, overcurrent relay, primary and backup (P/B) relays.

Impedance of line Voltage at bus or per unit).

NOMENCLATURE

.

after fault occurrence (in volts

Short circuit current of line (in amperes).

due to fault F

PSM

Plug setting multiplier.

New admittance matrix of the network.

TSM

Time setting multiplier.

Number of buses in the network.

Maximum value of load current (in amperes). Critical fault point

I. INTRODUCTION

Coordination time interval (in seconds). Length of line

.

Fault current detected by backup relay (in amperes). Fault current detected by primary relay (in amperes). Operation time of backup relay for the fault at point x (in seconds). Operation time of primary relay for the fault at point x (in seconds). Manuscript received November 03, 2013; revised April 18, 2014; accepted June 06, 2014. Date of publication March 10, 2015; date of current version March 20, 2015. Paper no. TPWRD-01249-2013. D. S. Alkaran, M. R. Vatani, and G. B. Gharehpetian are with the Electrical Engineering Department, Amirkabir University of Technology, Tehran 15914, Iran (e-mail: [email protected]; [email protected]; [email protected]). M. J. Sanjari and A. H. Yatim are with the Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Johor Darul Takzim, 81310 UTM, Johor, Malaysia (e-mail: [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.2014.2330767

D

UE TO a sudden increase of the current magnitude during a fault occurrence, the current has been traditionally used for fault detection. The protection of electrical equipment against the fault current is the first task of the protection system. Due to its simple implementation and low cost, the overcurrent protection is the most common method for the protection of power transmission lines [1]–[3]. Different types of this relay, such as constant time, inverse time, instantaneous, and combinatorial overcurrent relays can be used for power transmission-lines protection depending on the network configuration (i.e., radial or interconnected) and its voltage level. The overcurrent relays are used as primary relays in distribution networks, while they are used as backup in subtransmission and transmission networks [4], [5]. The input and output signals of the overcurrent relay are current and command, respectively; the former is measured by the meter connected to the current transformer and the latter is sent to the normally closed circuit breaker (CB). The overcurrent relay settings are PSM and TSM, by which the necessary values of the time and current for the operation of the relay are determined [6]–[8]. According to the time–current characteristic curve, the overcurrent relays are divided to constant current,

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ALKARAN et al.: OVERCURRENT RELAYS COORDINATION IN INTERCONNECTED NETWORKS

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TABLE I PARAMETERS OF DIFFERENT TYPES OF INVERSE-TIME RELAY

Fig. 1. Close-in fault.

TABLE II VARIATION RANGES OF PSM AND TSM

Fig. 2. Fault

constant time, and inverse-time relays. The inverse-time relay is classified into inverse, very inverse, and extremely inverse. The relationship between the time and current in these relays can be expressed by the following equation according to the IEC 60255 standard: (1) where and are two constants determined based on relay type, as listed in Table I. In analog relays, PSM is set proportional to the . The range of variations of the PSM and TSM in analog relays is listed in Table II. It should be noted that the mentioned parameters can be continuously set in digital relays [9]. Ideally, it is expected that the overcurrent relays remain coordinated for different operation conditions of the power system, all types of faults, single or multi-contingencies, and changes in the network topology. Moreover, the difference between the operation time of primary and backup (P/B) relays should be greater than the coordination time interval (CTI) for each pair of the main and backup relays, and for faults occurring in different places in the protection zone of the primary relay [10]. It is impractical to consider all of the different conditions of the network structure and all types of faults in the protection zone of the primary relay to coordinate the relays in the power system. Therefore, critical fault conditions are considered for the coordination of overcurrent relays in interconnected networks [11]. These conditions are the worst cases from the CTI point of view such that the minimum value between the P/B relays operational time is reached. Based on this definition, the P/B relays are coordinated for a fault at the critical point. Due to neglection of some fault cases in the overcurrent relays coordination, it is possible that the difference between the operational time of P/B relays is less than CTI for faults located in the protection zone of the main relay. The radial networks only have one critical fault point (CFP) caused by the symmetric three-phase fault occurrence at the front of the primary relay called the close-in fault as shown in Fig. 1 [12], [13]. This method is not effective for interconnected networks. Using six pairs fault currents is one of the most usual methods for the determination the fault critical current [14]. This method selects six pairs of fault currents for each P/B relay pair, including close-in faults, far-end faults, circuit-breaker failure,

.

arc resistances, instantaneous trips, and line outages [12], [14]. However, the CTI index may not be satisfied for some faults location in the protection zone of the main relay. In this paper, an analytical method is proposed to calculate the impedance matrix of the network in-fault condition. Accurate CFP is then determined based on the network impedance matrix. Using the determined critical point, the validity of relays coordination presented in [9] is assessed. The minimum value of the difference between the operation time of P/B relays set by [9] is calculated and compared with the minimum-allowable value of CTI. It is shown that the differences between operational times of P/B relays are not lower than the minimum value of CTI in several pairs of P/B relays. Therefore, the algorithm presented in [9] is improved by using the analytically determined CFP for the optimal setting and coordination of overcurrent relays. The main contributions of this paper are as follows. • A novel approach is proposed in this paper to calculate the impedance matrix of the network in-fault condition. • The current can be calculated in each point of the network in-fault condition based on the impedance matrix. • CFP is calculated based on an analytical method. In the existing methods, some special points are checked to test the relays coordination (e.g., close-in, far-end, and, in the general case, the six-pair currents. As shown in this paper, in some cases, these points are not CFPs and the P/B relays are not coordinated. • The relay coordination algorithm is modified considering the CFP accurate determination. II. PROBLEM FORMULATION A part of an interconnected transmission system is shown in Fig. 2, which has a pair of P/B relays protecting the lines. and are the P/B relays located at line and line , respectively. The difference between the operational time of P/B relays should be at least equal to CTI for all faults located at line . Assume that fault occurs at a point on the line with the distance of from bus . The following optimization problem should be solved to find the CFP:

subject to:

(2)

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Fig. 4. Adding hypothetical bus X to the network.

(8)

(9)

Fig. 3. Flowchart of the proposed algorithm for setting and optimal coordination of overcurrent relays in interconnected networks.

The characteristic curve of the overcurrent relay and its time and current settings determine the operational time of the relay in terms of fault current. In this paper, characteristic curves of the overcurrent relays are stated by the following equation: (3)

Now, assume that fault occurs on line at a point with a distance of from bus . In the proposed method, a hypothetical bus is considered at the fault location to update the impedance matrix for the calculation of fault currents by using . The fault location at each line of the network can be continuously changed and in each condition, a new should be calculated. The calculation of the impedance matrix is a complicated algorithm and has a large amount of computational burden [15]. Therefore, it is impossible to calculate the impedance matrix of the network for infinite conditions of fault occurrence. In this paper, a new method is proposed for the modification of the impedance matrix for all fault conditions. A. Calculation of New Impedance Matrix of the Network

To solve the optimization problem stated by (2), the values of and should be calculated in terms of and other parameters of the network. In this paper, this equation has been solved by the Optimization Toolbox of MATLAB software. In an interconnected system, the following objective function is used in the optimization procedure for the optimal setting and coordination of overcurrent relays [9]: (4) The genetic algorithm (GA) in MATLAB software is used to solve (4) based on the proposed algorithm shown in Fig. 3. III. ANALYTICAL METHOD FOR THE CALCULATION OF FAULT CURRENT Assume that a three-phase fault occurs at bus in an interconnected system. The following equations can be written to calculate the fault current in the lines and voltage at the network buses: (5) (6) (7)

A new method is presented in this section to calculate the new impedance matrix of the network. In this section, it is assumed that the of the network in the normal condition (base case) is known and we want to calculate the for the network, if bus is added at the point on line with the distance from bus (Fig. 4). In general, to calculate the in the , a unit current source is injected to bus . The measured voltage at bus is equal to when other sources are disconnected from the network. The following issues should be considered: 1) The value of for and is not changed by adding the new bus to the line , because the hypothetical bus at line does not affect the voltage value at bus . 2) To calculate for , a unit current source is injected to the bus and the following equation is written to calculate :


According to (15), matrix cept for elements , , lowing equation:

TABLE III TEST SYSTEM PARAMETERS

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is similar to the old ex, and . Consider the fol-

(18) where is the unity matrix. Therefore, the following equation is obtained by multiplying the last row of and the last column of : (19) By using (13), this equation can be written as follows: (10) (20) The values of

and

Using some simplifications, we obtain the following equation for :

are calculated as follows: (11)

In this condition, the voltage of the line varies linearly along it from bus to bus . The value of the voltage at bus with distance from bus is calculated, as follows: (12) can be calculated by using (12) in the case of injecting a unit current source at bus . This value of is equal to because no current is injected at other buses. Equation (12) can be written by substituting the variables stated in (11), as follows:

(21) B. Calculation of Short-Circuit Current for P/U Relays In the previous section, a new method has been proposed for the calculation of the new impedance matrix of networks after the addition of bus along a line. If a fault occurs at bus , the fault current flowing through P/B relays is calculated by using the modified . Using (12) and (21), the short-circuit current can be calculated as follows:

(13) 3) To calculate , we use the concept of admittance matrix of the network . By adding the new bus to the network, the of the system can be written as follows by using the rules of construction [16]

(22)

(14) where

and

are (23) (15) Otherwise (16) (17)

(24)

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TABLE IV SHORT-CIRCUIT CURRENT FLOWING THROUGH THE P/B RELAYS

TABLE VI DIFFERENCE BETWEEN OPERATIONAL TIME OF P/B RELAYS IN SCENARIO 1

TABLE V TSM AND PICKUP OF RELAYS IN DIFFERENT SCENARIOS TABLE VII CFP FOR EACH PAIR OF RELAYS (P/B RELAYS)

The short-circuit currents flowing through P/B relays in terms of and other given parameters of the network are calculated according to (23) and (24). The calculated short-circuit currents are used in (4) to optimize the objective function in order to coordinate the P/B relays. IV. SIMULATION RESULTS In this section, the proposed method is simulated to determine the validation of the coordination procedure presented in [9]. Then, the relays will be accurately coordinated by using the proposed algorithm as shown in Fig. 3. Fig. 5 shows the test system, including six buses, two generators, and two transformers. The voltage level of the network and generators is 150 and 10 kV,

respectively, and the network data are listed in Table III. The type of fault is symmetrical three-phase fault. A. Scenario 1 In [9], the overcurrent relays have been coordinated using the close-in pair fault currents. It has been simply assumed that the CFP is located in front of the primary relay. According to this method, the fault currents flowing through P/B relays are given in Table IV. The pickup and discrete TSM values of relays in this scenario are listed in Table V. The difference between the


Fig. 5. Test system.

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Fig. 7. Operational time of P/B relays and their difference in terms of fault location in the protection zone of the primary relay for a pair of relays (5,7).

TABLE VIII RELAYS NOT SATISFYING MINIMUM CTI

Fig. 8. Operation time of P/B relays and their difference in terms of fault location in the protection zone of the primary relay for a pair of relays (1,14).

Fig. 6. Fault current flowing through the backup relay (r5) due to a fault located at the protection zone of primary relay (r7).

operational time of P/B relays calculated based on (1) is listed in Table VI. By using (23) and (24) and values of TSM in the third column of Table V, the CFP for each pair of relays (P/B relays) is determined. Table VII lists the CFP for each pair of relays in the last column. 0 and 1 indicate close-in and far-end faults, respectively. It is obvious that the CFP for all pairs of relays is located at the front of the primary relay except the pair of relays (1,14), (9,14), and (5,7), in which the CFP is located at the far end of the primary relay . Table VIII gives the value of for these pairs of relays using a pair of critical currents of Table VII. Although the CFP for some of the pairs of relays is located at the front of the primary relay, the perfect coordination may not be established for them. This occurs due to the fact that the value of the fault current flowing through the backup relay is less than its setting (listed in the second column of Table V) for some fault locations at the front of the primary relay, and it will not operate. The example of this problem regarding the

pair of relays 5 and 7 is shown in Fig. 6. This figure shows the fault current flowing through the backup relay in terms of (the fault location varying from the front of the primary relay to the next bus). As shown in Fig. 6, it is obvious that the current flowing through the backup relay is less than the relay setting in some points (around 0.5). The operational time of P/B relays and their difference for the pair of relays (5,7) in the case of occurring fault in the protection zone of the primary relay is shown in Fig. 7. This figure shows that the operation time of the backup relay is less than the operation time of the primary relay for the fault occurring at the end of the protection zone of the primary relay, meaning that the coordination between these relays is not perfect. The CFP for this pair of relays is 1 (far end of the primary relay). Figs. 8 and 9 show a similar problem for pairs of relays (1,14) and (9,14), respectively. B. Scenario 2 In this section, the problem of relay coordination is solved by optimizing (4), using GA and Optimization Toolbox of are selected the same as [9] MATLAB software. , , and and in the procedure of determining the appropriate setting of relays, the accurately calculated CFP is used. It is expected to have better results than [9], because the accurately determined CFP is used in this paper. The PSM and TSM are assumed to be discrete to make the results comparable with [9]. By minimizing (4), the TSM of relays is determined as listed in

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TABLE X FOR BACKUP RELAYS AND THE DIFFERENCE BETWEEN THE OPERATIONAL TIME OF P/B RELAYS IN SCENARIO 3

CFP

Fig. 9. Operation time of P/B relays and their difference in terms of fault location in the protection zone of the primary relay for a pair of relays (9,14).

TABLE IX CFP FOR BACKUP RELAYS AND THE DIFFERENCE BETWEEN THE OPERATIONAL TIME OF P/B RELAYS IN SCENARIO 2

are positive and small. This means that the coordination between relays is perfect by using the proposed method. V. CONCLUSION

the fourth column of Table V. The difference between the operational time of P/B relays is listed in Table IX. In this table, the CFP of backup relays is also calculated. C. Scenario 3 In this section, the proposed algorithm for the optimal coordination of relays is simulated on the test network shown in Fig. 5. The pickup values of relays are considered as listed in Table V. It is assumed that the TSM of relays varies continuously from 0 to 1. By solving the optimization problem stated in (4), the TSM of relays is determined which are listed in the fifth column of Table V. The values of the for all pairs of relays, the fault exact critical points, and critical currents are listed in Table X. As can be seen in Table X, the CFP for primary relays 7 and 14 is far end from primary relays , and all values of the

This paper has presented a new analytical method for determining the exact CFP. Also, a new method has been introduced for calculating the fault currents flowing through P/B relays. This method quickly modifies the impedance matrix of the network using the analytical approach. Using the modified impedance matrix of the network, the exact CFP has been determined. Finally, the accurate settings of P/B relays have been calculated by solving the optimization problem. In contrast with previous methods, which are based on the empirical CFP, such as the close-in point, this algorithm uses the exact CFP for the coordination of overcurrent relays. Therefore, the coordination is satisfied in all protection zones of the primary relay. Based on simulation results, it is shown that in comparison with other methods, the proposed method results in accurate settings. REFERENCES [1] H. A. Abyaneh, M. Al-Dabbagh, H. K. Karegar, S. H. H. Sadeghi, and R. A. J. Khan, “A new optimal approach for coordination of overcurrent relays in interconnected power systems,” IEEE Trans. Power Del., vol. 18, no. 2, pp. 430–435, Apr. 2003. [2] P. P. Bedekar and S. R. Bhide, “Optimum coordination of directional overcurrent relays using the hybrid GA-NLP approach,” IEEE Trans. Power Del., vol. 26, no. 1, pp. 109–119, Jan. 2011. [3] A. S. Noghabi, H. R. Mashhadi, and J. Sadeh, “Optimal coordination of directional overcurrent relays considering different network topologies using interval linear programming,” IEEE Trans. Power Del., vol. 25, no. 3, pp. 1348–1354, Jul. 2010. [4] R. M. Chabanloo, H. A. Abyaneh, S. S. H. Kamangar, and F. Razavi, “Optimal combined overcurrent and distance relays coordination incorporating intelligent overcurrent relays characteristic selection,” IEEE Trans. Power Del., vol. 26, no. 3, pp. 1381–1391, Jul. 2011.


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Davood Solati Alkaran received the B.Sc. degree in electrical engineering from Sharif University of Technology, Tehran, Iran, and the M.Sc. degree in electrical engineering from Amirkabir University of Technology, Tehran, in 2012. His research interests are power system protection, optimization, distributed generation, and power system modeling.

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Mohammad Reza Vatani received the B.Sc. degrees in electrical engineering from the University of Tehran, Tehran, Iran, in 2010 and the M.Sc. degree in electrical engineering from Amirkabir University of Technology, Tehran, in 2013. His research interests are power system analysis, distributed generation, and smart grids.

Mohammad Javad Sanjari received the B.Sc., M.Sc., and Ph.D. degrees in electrical engineering from Amirkabir University of Technology, Tehran, Iran, in 2006, 2008, and 2013, respectively. His research interests are power system security assessment, distributed generation, and smart grids.

Gevork B. Gharehpetian received the B.S. degree (Hons.) in electrical engineering from Tabriz University, Tabriz, Iran, in 1987, the M.S. degree (Hons.) in electrical engineering from Amirkabir University of Technology (AUT), Tehran, Iran, in 1989, and the Ph.D. degree (Hons.) in electrical engineering from Tehran University, Tehran, in 1996. As a Ph.D. student, he has received a scholarship from DAAD (German Academic Exchange Service) from 1993 to 1996 and he was with the High Voltage Institute of RWTH Aachen, Aachen, Germany. He has been Assistant Professor at AUT from 1997 to 2003, Associate Professor from 2004 to 2007, and Professor since 2007. He is the author of more than 700 journal and conference papers. His teaching and research interest include power system and transformers transients and power-electronics applications in power systems.

Abdul Halim Yatim (M’89–SM’01) received the B.Sc. degree in electrical and electronic engineering from Portsmouth Polytechnic, Portsmouth, U.K., in 1981, and the M.Sc. and Ph.D. degrees in power electronics from Bradford University, Bradford, U.K., in 1984 and 1990, respectively. Since 1981, he has been a member of the Faculty of Electrical Engineering at the Universiti Teknologi Malaysia, and is currently a Professor and was Dean of the Faculty in 2010–2012. He was a Commonwealth Academic Fellow during 1994–1995 at Heriot Watt University, Edinburgh, U.K., and a Visiting Scholar at the Virginia Power Electronics Centre, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA, in 1993. His research interests include renewable/alternative energy, power-electronics applications, and drives. Dr. Yatim is a Fellow of the Institution of Engineers Malaysia and a Registered Professional Engineer with the Malaysian Board of Engineers. He was the first Chapter Chairman of the Malaysian section of the IEEE Industrial Electronics/Industry Applications/Power Electronics joint societies formed in 2003.