UPGRADING SUBSTATION RELAYS TO DIGITAL RECLOSERS AND THEIR COORDINATION WITH SECTIONALIZERS

Transcription

1 UPGRADING SUBSTATION RELAYS TO DIGITAL RECLOSERS AND THEIR COORDINATION WITH SECTIONALIZERS 1 B. RAMESH, 2 K. P. VITTAL Student Member, IEEE, EEE Department, National Institute of Technology Karnataka, Surathkal, India Senior Member, IEEE, EEE Department. National Institute of Technology Karnataka, rbant96@gmail.com, Surathkal, India Abstract Fast and reliable transfer of the system could be made automatic with the use of automatic tie breaker switch. Bus tie breaker considered is being manually controlled to supply the loads when one of the two sources is not available. This paper presents the PSCAD modeling of control circuits of reclosers and sectionalizers in the presence of the bus tie breaker. As the load connected changes when the bus tie breaker status is changed, automatic change in recloser setting is proposed. The bus system is modeled in PSCAD and the protection coordination between recloser and sectionalizer is achieved for different cases. Index Terms PSCAD, bus tie breaker, recloser, sectionalizer, distribution system I. INTRODUCTION Critical electric power distribution systems need automatic operation. Transfer of system from existing power source to a different source need to be done reliably. This improves the reliability goal of the system and the connected facility as well as speed of operation. Automatic transfer of system need to be given more importance than the present practice. Otherwise there could be catastrophic consequences related to reliability of the system [1]. There are several major concerns in the protection of complex distribution substations equipped with a bus-tie breaker and served by two transformers. For a fault in the transformer zone, the bus served by the faulted transformer experiences an outage if the bus-tie breaker is operating normally open. With manual operation, this outage lasts until a crew manually reenergize the bus by closing the bustie breaker. With this, all the loads are supplied through the second transformer. It is possible to deploy intelligent electronic devices (IEDs) and obtain an automatic bus transfer scheme as per requirement [2]. Self monitoring and automatic bus transfer which is having the ability to monitor the abnormal conditions and which transfers the power source from one to another is proposed in this paper. The paper also outlines the automatic recloser and sectionalizer coordination considerations during automatic transfer of the system through the bus tie breaker of a local substation. II. PROTECTION COORDINATION DURING AUTOMATIC BUS TRANSFER The electrical system considered consists of two 33 kv/11kv transformers are supplying six feeders through two buses and one bus coupler (Fig.1). One overhead line of 1mile length is connecting the 33kV/11kV substation with 11kV/440V substation located near the hostels. The study is about replacing the existing two substation relays with digital reclosers and to make the bus transfer automatic through the bus tie breaker. The recloser and sectionalizer coordination during two positions of the bus tie breaker will be analyzed. Reclosers are located in the main feeders to protect distribution systems from temporary faults and sectionalizers or fuses are used at the beginning of laterals and sublaterals to protect the system from permanent faults. Fig.1. Power distribution system under study 85

2 The bus tie breaker is ON only when one substation (SS) transformer or its supply line is off. For instance, if transformer T1 is off, transformer T2 will be supplying all the loads through the bus coupler. Then the relay setting has to change for the recloser R2 connected to this transformer T2. The major part of the study is for feeder 1 faults as it is the overhead line. The maximum load current flowing through feeder 1 = 1.25x 1.41x 10 6 / (1.732 x 11 x 1000) = A Based on this the feeder 1 conductor data are indicated in Appendix I. Three possible fault scenario cases are considered in the forthcoming sections. The scenario of the tie breaker switch is in OFF position, and both sources are available is considered under case 1 as normal condition. When the source 2 is not available, and source 1 is supplying Load 1 and Load 2 with the tie breaker switch in ON condition is studied under case 2. When the source 1 is not available, and source 2 is supplying both loads with the tie breaker switch is in ON position is studied under case 3. The need for these three cases is that the recloser settings need to be changed for these three cases. LINE CALCULATIONS: Transmission line side base current = = A Generator side base current = = A a) 3 Fault at 10% of feeder length b) 3 Fault at 100 % length of feeder Z Total = j I sh p.u = x I sh = 1235 A Max value of I sh = A c) L-L fault at the end of line Fig.4: Equivalent circuit for AB fault at 100% length I fb = p.u Fault current = A Fault current + Load current = A Maximum value = 1750A III. RECLOSER SETTING Fig.2: Equivalent circuit for ABC fault at 10% length For generator, Impedance = 1+j1 Base impedance = 33 2 /100 = Ω p.u impedance = (1+j1)/10.89 = ( j 0.091) For transformer, Reactance = j 0.06 x 100/5 = j 1.2 p.u Z Total = j Ω ے = ے (1.3933) / 0 ے = 1 p.u I ے = I sh = x = 3767 A Max Value of I sh = 5.18kA Recloser will sense and interrupt fault currents and reclose automatically while attempting to reenergize the line. If the fault is temporary in nature, it will be cleared when the line is de-energized [4]. Whenever there is a detection of fault, the recloser opens its contacts for a pre-programmed period. After this dead time, it will close automatically. If the fault is permanent, the recloser will finally remain open which is called as recloser lock-out. Case 1: Normal condition During normal conditions, transformer T 1 is supplying power to combined load 1 and transformer T 2 is supplying power to combined load 2. The load 2 comprises of local loads fed by underground cables. The recloser model has fault detecting, interrupting and reset unit. The fault detecting unit will sense faults. The fault interrupting unit has four shots, each operating in different modes as specified 86

3 by the time current characteristic curves. The time current characteristics are represented by the curves A, B, C, and D. s B and C are of inverse time overcurrent type. The recloser pick up current is selected as two times the maximum load current. The first trip signal is by fault detection using ANSI 50 element from the instantaneous blocks. The second and consecutive trip signals are by fault detection using ANSI 51element (for curve B and C). Relay settings of recloser 1 for different curves are as indicated in table I. The recloser 1 control flow diagram is indicated in Fig. 7. The control signal R1 is generated using curve A, B, C and D. TABLE I RECLOSER SETTINGS FOR CASE 1 The recloser settings for this condition are given in table IV. The fault current levels in this case are listed in table V. One current waveform is indicated in Fig.5 Case 3: Source 1 unavailable In this condition, source 2 is supplying all the loads through transformer 2. All the load currents will flow through the recloser 2. The fault current levels in this case are listed in table VI. One current waveform for this case is indicated in Fig.6 Fig.5. Recloser 1 currents for abc fault at feeder 1 (case 2) A B C D TABLE II RECLOSER 2 SETTINGS FOR CASE 1 Recloser 2 settings I pickup (ka) I pickup (ka) TDS (s) type Instantaneous IEEE Very inverse IEEE Extremely inverse Instantaneous TABLE III FAULT CURRENTS FOR CASE 1 Fig. 6. Recloser 2 currents for ABC fault (case3) Case 2: Source 2 unavailable In this condition, source 1 is supplying all the loads through transformer 1. The maximum load current through the recloser 1 = 274A. Fig. 7. Recloser control 87

4 TABLE IV RECLOSER 1 SETTINGS FOR CASE 2 AND RECLOSER 2 SETTING FOR CASE 3 TABLE V FAULT CURRENTS FOR CASE 2 Fig. 9. Tie breaker control flow diagrm TABLE VI FAULT CURRENTS FOR CASE 3 The bus tie breaker is normally in open position. It is closed only when any one source is not available. When the recloser 1 is open, the tie breaker switch will be ON connecting the second source to supply all the loads. Similarly when the recloser 2 is open, all the loads are supplied by source 1. (Fig.9). The Recloser settings will be automatically changed with the change in tie breaker switch condition (Fig. 10). Fig.10. Recloser 1 curve setting flow chart Fig.11. Sectionalizer control flow chart 88

5 ii) A fault of 14 s duration is applied at 2s of simulation time. The recloser 1 signal is indicated by Fig. 13. As the fault is cleared by the opening of the sectionalizer after the third shot, the recloser will finally close as the fault is cleared. Fig.12. Recloser 1 status for a fault of 4s on feeder 1 (Case1) With the tie breaker control, during any fault in source 1 circuit, source 2 will take over and supply both the loads. CONCLUSION Fig.13. Recloser 1 status for fault of 14s duration IV. SECTIONALIZER CONTROL The sectionalizer does not interrupt fault current. Instead it counts the number of operations of the interrupting device upstream and opens while the interrupting device is open [3]. If the fault is temporary and cleared before the count reaches the programmed count, the unit does not operate and after the reset time expires, the count is reset. If the fault is permanent, the unit will see multiple instances of fault current followed by recloser opening and in each instance, the count will be incremented. After detecting the preset number of counts, the sectionalizer will open. The sectionalizer presented will open after the third trip of the recloser and clears the fault (Fig. 11).By this, the recloser could close the contacts. But if the fault is not cleared, recloser will remain open after the fourth shot. V. SIMULATION OF OPERATING CONDITIONS The coordination setting selected here is such that the first recloser opening takes place with instantaneous curve A. There will be a dead time of 0.5s after this trip. The second and third trips are following the curve B and C respectively with dead time delays of 5s and 10s respectively after the trip. If the fault is not cleared during this time, the last trip is following the instantaneous curve and recloser will remain open. Simulation study was conducted for the three cases. Test results for case 1 are described below. i) A fault of 4s duration is applied at 2s of simulation time. As the fault is cleared before the preset count, sectionalizer will not open (Fig. 12). The benefits of the proposed method are: i) Automatic transfer of load to second source whenever there is a fault in the source 1 circuit. This reduces the operation time of system transfer. ii) Automatic change in the recloser settings when the tie breaker status changes. iii) Coordination between recloser and sectionalizer in the three scenarios. An intelligent recloser with self configuring features is proposed. This type of reclosers and sectionalizers will support protection feature, much desired in smart grid environment. APPENDIX I REFERENCES TABLE VII FEEDER 1 CONDUCTOR DATA [1] P.E.B. Brown, and J. Guditis, Critical power automatic transfer systems- design & applications, Square D Critical Power Competency Center, Revision 0, 10/06, Schneider Electric, [2] T. Zhao, L. Savior, and C. Wester, Advanced bus transfer and load shedding applications with IEC 61850, GE Digital Energy Multilin. [3] V.A.S. Rones, and K.P. Vittal, Modeling of recloser and sectionalizer and their coordination using PSCAD, IEEE Conference on Circuits, Power and Computing Technologies, 2/13, pp , (2013). [4] IEEE guide for automatic Reclosing of Line circuit Breakers for AC Distribution and Transmission lines, IEEE std C