Impact of Bridge type Fault Current Limiter on Power System Transient Stability

Transcription

1 ELECO 11 7th International Conference on Electrical and Electronics Enineerin, 1-4 December, Bursa, TUKEY Impact of Bride type Fault Current Limiter on Power System Transient Stability Seyed Behzad Naderi 1, Student Member, IEEE, Mehdi Jafari 1, Student Member, IEEE and Mehrdad Tarafdar Hah, Member, IEEE 1 Department of Electrical Enineerin, Sarab Branch, Islamic Azad University, Sarab, IAN s.b.naderi@ieee.or, m.jafari@ieee.or Faculty of Electrical and Computer Enineerin, University of Tabriz, Tabriz, IAN tarafdar@tabrizu.ac.ir Abstract In this paper, transient stability improvement usin bride type fault current limiter (FCL) is presented in sinle machine infinite bus (SMIB) system with a double circuit transmission line. Three sinle-phase sets of the proposed FCL are installed at the beinnin of feeder. The proposed FCL inserts an inductance and a resistance in the fault current pass. The insertion inductance and resistance not only limits the fault current level in an acceptable value but also improves transient stability of power system by consumin excessive enery of synchronous enerator durin fault. To reach maximum transient stability, the optimal resistor value of the proposed FCL is calculated. Analytical analysis and simulation results usin PSCAD/EMTDC software are presented to show the current limitin future and transient stability enhancement usin the proposed FCL in SMIB. Keywords-transient stability improvement; fault current limiter; optimal resistor; semiconductor switch. 1. INTODUCTION Two of the most important desin considerations for power systems are transient stability and dampin of electromechanical modes of sustained oscillation. These two desin considerations have assumed even reater importance in the wake of recent interconnection blackouts in the U.S., Canada, and Europe. So, the transient stability plays an important role for maintainin security of power system operation [1-3]. Due to the importance of transient stability improvement, different methods are introduced in literature for this purpose, such as power system stabilizer (PSS), breakin resistor, superconductin manetic enery storae (SMES) and flexible AC transmission (FACTs) devices [4-8]. As another solution, fault current limiters (FCLs) can improve the transient stability of the power system by suppressin the level of fault currents in a fast and effective manner. The FCL which is capable of consumin the active power can be applied to the enhancement of the power system transient stability by absorbin the acceleratin power of synchronous enerator durin fault. On the other hand, optimum value of FCL s resistance is important from the transient stability point of view [9-1]. One roup of these structures is -type superconductin FCLs (SFCLs). The SFCLs limit the fault current by usin resistance and consume the excessive enery of synchronous enerator durin fault [1-14]. However, the FCLs which use superconductor (SFCLs) have two main problems. The main problem is hih construction and hih maintenance cost of superconductors. So, these devices are not commercially available. In addition, the resistance of SFCL is not constant durin the fault due to its quenchin characteristics [1]. So, determination of the optimal resistor value of these FCLs is difficult slihtly. In [14] and [15], the transient stability improvement of the power system is presented by use of the SFCL in parallel with a resistor in series with a ZnO device. The mentioned FCL s impedance is inductive type (because of usin superconductor) and resistive type. Usin inductance can limits ac components of fault current better than -type of FCLs. In these structures, two cases must be considered: 1) Insertion the optimal resistor value durin fault ) Hih construction and maintenance cost of superconductors Above mentioned cases are important from both the transient stability and commercial point of view. In this paper, the proposed FCL inserts an inductance and a resistance to improve the transient stability of power system in addition to fault current limitin. The inductive part of the proposed FCL s impedance limits ac components of fault current and the resistive part not only limits the fault current, but also consumes the excessive enery of the synchronous enerator without usin any superconductor. So, the proposed FCL can improve transient stability of the power system. To calculate the optimal resistor value to reach maximum transient stability, analytical analysis is presented in detail. Simulation results usin PSCAD/EMTDC software is provided considerin the proposed FCL with the optimal resistor value.. POWE CICUIT TOPOLOGY OF THE POPOSED FCL AND ITS OPEATION Fi. 1 shows the power circuit topoloy of the proposed FCL which is composed of two followin parts: 1. Bride part that includes a diode rectifier bride, a small dc limitin reactor ( L ), a semiconductor switch (IGBT or GTO), a free wheelin diode ( D 5 ).. Main part: shunt branch which consists of a resistor and an inductor ( sh + jωlsh ). In the normal operation of power system, the semiconductor switch is ON and the L dc is chared to the peak of the line current, therefore behaves as a short circuit. Usin dc 148

2 ELECO 11 7th International Conference on Electrical and Electronics Enineerin, 1-4 December, Bursa, TUKEY (PCC), the output power of synchronous enerator reaches near to zero. The proposed FCL inserts the impedance which includes the resistor and the inductance in fault current path. So, the proposed FCL not only restores the PCC voltae and the transmitted power of healthy line, but also ensures the transient stability of the synchronous enerator by consumin excessive enery of fault. Fi. 3 shows star-delta equivalent circuit of Fi.. In this condition, the enerator current can be express by Eq. (). ( δ α ) (( δ ) α1) I = E Z + E V Z () b a Fi. 1. Power circuit topoloy of the proposed FCL semiconductor devices (the diodes and the semiconductor switch) and small dc reactor, cause a small voltae drop on the proposed FCL that it is neliible. So, the proposed FCL does not affect the normal operation of the power system. Because of small value of L dc, it can be desined with air core to prevent its saturation. As a fault occurs, the line current beins to increase, but the L limits its increasin rate and protects semiconductor switch dc aainst severe di dt at the beinnin of fault occurrence. When the current reaches to the maximum permissible fault current, I m, which is specified by operator, control system of the semiconductor switch turns it off. So, the bride retreats from feeder and shunt impedance enters to the faulted line and limits fault current and consumes excessive enery of synchronous enerator durin fault. At this moment, the free wheelin diode dischares L dc. In fact, free wheelin diode is used to provide free route for dc reactor current when the semiconductor switch is off. After fault removal, the semiconductor switch turns on aain and the proposed FCL returns to the normal state. 3. TANSIENT STABILITY AND OPTIMAL ESISTO CALCULATIONS The power system of Fi. is used for transient stability calculations. The proposed FCL is installed at the beinnin of the most exposed power line. In addition, parallel lines have same characteristics. In the normal operation of power system, the output power of synchronous enerator can be expressed as follow: ( ) ( ) ( ( ω ) ( ( ω ))) Za = b + c + bc a Zb = a + c + ac b a = Z + j + j L X + j X + L b = X L ( sh + j( X L + ωlsh )) c = jx + j + j L X + j X + L X = X d + X t F sh sh L sh L sh ( ( ω ) ( ( ω ))) sh sh L sh L sh Z F and X d are fault impedance that is zero in three phase fault conditions approximatly and unsaturated transient reactance, respectively. The output power of enerator, P f, can be computed durin fault as follow: ( EV Za ) ( ) ( ) f = ( δ ) = a cosα1 + b cosα P real I E E Z E Z + sin( δ + α π ) 1 It is obvious that P f depends on the impedance of the proposed FCL. So, to reach the maximum transient stability or minimum rotor speed swin, the optimal resistor value of the proposed FCL, sh, must be calculated. In next section, it is shown that rotor speed oscillation will be minimized for the optimum resistor value. (3) (4) ( ) sin P = EV X δ (1) where: E : MS line to line synchronous enerator voltae; V : MS line to line infinite bus voltae; X : Total reactance ( X = X + X + X ); X d t d t L : Unsaturated reactance of enerator; X t : Transformer reactance; : Line reactance; X L δ : Load anle. Three phase faults are the worst fault conditions which makes the transmitted power zero in faulty line, approximately. So, the synchronous enerator becomes unstable, probably. Considerin Fi., if three phase fault occurs in point of common couplin Fi.. The power system with the proposed FCL Fi. 3. Equivalent circuit of Fi. 149

3 ELECO 11 7th International Conference on Electrical and Electronics Enineerin, 1-4 December, Bursa, TUKEY To calculate the optimal value of sh, the active power of enerator should be equated with pre-fault condition active power, durin fault. The consumed active power of the proposed FCL, P fcl, can be expressed by Eq. (5). P fcl = VPCC sh sh + ω Lsh where V PCC is the point of common couplin voltae (Line to Line, MS). Because of same characteristics of parallel lines, transfer power of each line is considered equal before fault. So, considerin Eq. (1) and (5), we have: As a result: V P EV = = sinδ X PCC sh sh + ω Lsh sh, opt 4 PCC + PCC ω sh (5) (6) V V P L = (7) P where sh, opt is the resistor optimal value of the proposed FCL. Considerin Eq. (7), a condition must be considered as follow: L sh PCC V < (8) ωp 4. SIMULATION ESULTS Transient stability improvement feature of the proposed FCL on the SMIB power system as Fi. is simulated by the PSCAD/EMTDC software and its results are presented in this section. Simulation parameters are as Table 1. A three-phase short circuit fault occurs at t=15s and continues.s (1 cycles of power system frequency). The current of faulted line (A phase current) without and with the proposed FCL are shown in Fi. 4. Fi. 4 shows this current without usin FCL. As it is observed, the faulted line current has a very lare manitude durin the fault and after fault removal; it experiences an instable condition because of the synchronous enerator instability. However, by usin the proposed FCL, the line current is limited durin the fault and therefore, it has not instability after fault removal (Fi. 4). Also, the dc side current of proposed FCL is shown in Fi. 4. In the normal operation, it is equal to the line current peak. As fault occurs, it is increased and after the semiconductor switch turnin off, it starts to dischare by the free wheelin diode. By the fault removal and semiconductor switch turnin on aain, the dc current follows the line current. Fi. 5 shows the faulted line transmitted power without and with the proposed FCL. Without usin FCL, it becomes zero durin fault and after fault it is distorted extremely (Fi. 5). By usin the proposed FCL, the power has neliible swin durin and after the fault. It is obvious that the power of faulted line is near to pre-fault condition active power durin fault. The output current of the synchronous enerator is shown in Fi. 6. This fiure shows the instability of enerator without usin the FCL and its stability by usin the FCL (Fi. 6 and 6, respectively). Fi. 7 shows these results for the voltae of enerator terminal. Without usin the FCL, it drops extremely and after fault, it becomes instable. However, its stability is ensured by usin the proposed FCL. Considerin Fi. 7a, the voltae of terminal enerator is restored durin fault which this condition causes that the healthy line is not affected by three phase fault in the faulted line and reliability of the power system is increased. Table 1. Simulation parameters 4 poles,38v(l-l),5hz Fi. 4. Active power (MW) DC current Faulted line current Faulted line current: without FCL and with the proposed FCL -. Sb = 4kVA, Generator Pm =.8 p. u. X 1.7 p. u. d =, Power X.394 p. u. system d = parameters 38/38 V, 5kVA, Transformer X.1 p. u. t = Infinite bus 4V, L-L MS, 5Hz Transmission lines X.7 p. u. L = FCL data sh = 7.7Ω, Lsh = 1mH, Ld = 5mH, V = V = 1V, I = 6A DF IGBT m 15

4 ELECO 11 7th International Conference on Electrical and Electronics Enineerin, 1-4 December, Bursa, TUKEY Active power (MW) Voltae (kv) Fi. 5. Transmmited power of the faulted line: without FCL and with the proposed FCL -.4 Fi. 7. The enerator terminal voltae: without FCL and with the proposed FCL Fi. 8 shows the rotor speed oscillation of the synchronous enerator caused by the fault. Without the proposed FCL, rotor speed is increased rapidly and it shows the enerator instability. The optimum value of resistor for the maximum transient stability is calculated 8 accordin to the calculations that is mentioned in section 3. The minimum swin of rotor speed is achieved for this optimum value. On the other hand, if the resistor of FCL has non-optimum value, enerator s speed oscillations become larer..6.4 Speed (rpm) =4 =16 =8(optimum value) Without FCL Fi. 8. otor speed oscillations of the enerator Fi. 6. Voltae (kv) Current of enerator terminal: without FCL and with the proposed FCL 5. CONCLUSION This paper proposes the FCL to improve the transient stability of synchronous enerator. The proposed FCL inserts an inductance and a resistance in the fault current path. The FCL s impedance limits the fault current in an acceptable value and resistive component of the FCL consumes excessive enery of fault. Three phase fault as the worst fault condition is considered and the optimal resistor value is calculated. It is shown that the maximum transient stability will be achieved for optimal resistor value. The simulation results usin PSCAD/EMTDC are presented. These results show that the proposed FCL is able to improve transient stability of power system in addition to current limitin capability. 6. EFEENCES [1] P. Kundur, Power System Stability and Control. New York: McGraw Hill, [] G. Andersson, P. Donalek,. Farmer, N. Hatziaryriou, I. Kamwa,P. Kundur, N. Martins, J. Paserba, P. Pourbeik, J. Sanchez-Gasca,.Schulz, A. Stankovic, C. Taylor, and V. Vittal, Causes of the 3 major rid blackouts in North America and Europe, and recommended means to improve system dynamic performance, IEEE Trans. Power Syst., vol., no. 4, pp , Nov. 5. [3] Naoto Yorino, Ardyono Priyadi, ironori Kakui, and Mitsuhiro Takeshita, A New Method for Obtainin Critical Clearin Time for Transient Stability, IEEE Trans. Power Syst., vol. 5, no. 3, pp , Au. 1. [4] Adam Dys ko, William E. Leithead, and John O eilly, Enhanced Power System Stability by Coordinated PSS Desin, IEEE Trans. Power Syst., vol. 5, no. 1, pp , Feb. 1. [5] Mohd. Hasan Ali, Toshiaki Murata, and Junji Tamura, Influence of Communication Delay on the Performance of Fuzzy Loic-Controlled Brakin esistor Aainst 151

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