Journal of Reliable Power - SEL

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7 After the remote terminal tripped, a reclose was expected after approximately 0.5 seconds. The fault was not permanent, but the reclose attempt failed. Oscillography from the local relay is shown in Fig. 17. The relay’s polarizing voltage has a memory of about one second. The CVT transient response and long-time decay are evident. This decaying voltage continues to feed the relay positivesequence (V1) memory and corrupts V1 magnitude and angle. When the line terminal attempts a reclose, the V1 memory has not reset, and inrush current from a tapped transformer load is present (see Fig. 18). The high current combined with the memory voltage error produces a Zone 1 distance element trip. IA IB IC VA VB VC Digitals 2000 1000 0 -1000 -2000 100 50 0 -50 -100 ZAB IA IB IC VA VB VC VA VB VC V1Mag V1Ang 100 0 -100 100 75 50 25 0 100 0 -100 Fig. 17. IA IB IC 2500 0 -2500 VA VB VC V1Mag V1Ang 1 2 3 4 5 6 7 8 9 10 11 Cycles Forward BG Fault as Viewed by Local Line Relay (Filtered Data) IA IB IC IAMag IBMag ICMag VA VB VC Fig. 19. 0 1 2 3 4 5 6 7 8 9 10 11 12 Cycles COMTRADE Replay of Event Data With V1 Memory Reset VIII. BLOWN PT FUSE In June 2000, both primary and backup relays at a 69 kV line terminal tripped for a potential transformer (PT) fuse problem. There was no system fault at the time of trip. The relays were different models but both provided distance and directional overcurrent functions. The primary relay was used in a DCB scheme and the backup relay provided step-distance protection and tripped instantaneously for Zone 1 faults. The false apparent impedance created by abnormal voltages and load currents caused the apparent impedance to encroach on the reach of the distance relays (see Fig. 20). The same bus potentials serve both relays. A B C Fuses IAMag IBMag ICMag 2000 1000 0 100 To Relay VA VB VC Digitals 0 -100 ZAB 3 1 1 1 1 1 1 1 PT Loose Fig. 18. 0 1 2 3 4 5 6 7 8 9 10 11 12 Cycles Raw Data From Local Relay During Reclose Event data from the reclose were converted to IEEE COMTRADE format and replayed into a similar relay in the lab. With no influence from corrupted V1 memory as in the real event in the field, the Zone 1 distance element (ZAB_1) did not trip (see Fig. 19). This was because the V1 memory was reset at the time the test was started and a fast recharge circuit instructed the relay to use actual measured V1. The CVT transient coupled with a fast reclose attempt caused the reclose failure. Fault detectors that supervise distance elements should be set above transformer inrush currents [8]. Reclose-open intervals must take into account CVT transient response and decay and V1 memory in relays. Fig. 20. PT Fuse Problem Because a blown fuse results in a loss of polarizing inputs to the relays, detection of this condition is desirable and was enabled in both relays (see Fig. 21). The event data show that the loss-of-potential (LOP) detection asserts after the phase distance element trips. In both relays, there is a three-cycle delay before the LOP element asserts for unbalanced conditions to ensure LOP does not block protective elements during a fault. 58 | Journal of Reliable Power
8 1_VA 1_VB 1_VC 2_VA 2_VB 2_VC 3_VA(kV) 3_VB(kV) 3_VC(kV) 25 0 -25 0 -50 25 0 -25 1_VA 1_VB 1_VC 2_VA 2_VB 2_VC 3_VA(kV) 3_VB(kV) 3_VC(kV) .020833 sec Digitals 3_LOP 2_OUT TP * 2_LOP * * 2_IN 52A * L 2_50P 2_21P 1 1_ZBC 4 2 1_OUT 1&2 1 1_LOP * * 1_IN 1&2 B 2 1_50P L 1_32 Q Q 50.50 50.55 50.60 50.65 50.70 50.75 Event Time (Sec) 23:31:50.588677 Fig. 21. Response of Primary (1), Backup (2), and Relay With Improved LOP Logic (3) to PT Fuse Problem In the primary relay, an LOP is detected when negativesequence voltage (V2) is greater than 14 volts secondary and negative-sequence current (3I2) is less than 0.5 amperes secondary [9]. In the backup relay, an LOP is detected when zero-sequence voltage (V0) is greater than 14 volts secondary and zero-sequence current (I0) is less than 0.083 amperes secondary [10]. Once asserted, LOP will block distance and directional elements that rely on healthy voltage signals. This event emphasizes that early LOP logic was designed to protect distance elements from misoperating for system faults that occurred some time after an initial LOP condition was detected. Overcurrent fault detectors, set above load, were used to prevent distance element misoperation when the LOP condition first occurred. In this event, the fault detector (50L) was picked up during balanced load flow. Ideally, fault detectors should be set above expected load currents and below minimum fault levels to ensure correct distance relay operation. Newer relays have LOP logic that operates based on the V1 rate of change versus the rate of change of currents. The new logic operates in less than 0.5 cycles, so distance element security is less dependent on the fault detector settings [11]. In Fig. 21, the response of the original relays are shown with that of a relay with improved LOP logic. The new relay LOP logic operates more than 1 cycle before any distance elements assert, ensuring this misoperation would not happen again. IX. DIRECTIONAL ELEMENT CHALLENGE In June 2007, 80 MW of generation was tripped offline because of an unknown cause (at that time). The generator stepup transformer was fed radially from a 138 kV tie line to a local utility for several seconds. The line was protected by a DCB scheme. A BG fault then occurred on the line. The relay at the generator end of the line saw this fault as reverse and sent a blocking signal to the utility terminal, delaying fault clearing. Event data from the relay at the generator end of the line are shown in Fig. 22. The phase currents are all in phase. Fig. 22. Fault Data From Relay A one-line and symmetrical-components diagram for the fault is shown in Fig. 23. With the generator breaker open, there is a pure zero-sequence source behind the relay. S – + Fig. 23. T Relay Z1S Z1T m • Z1L (1–m)Z1L Z1R Relay Z2S Z2T m • Z2L (1–m)Z2L Z2R Relay Z0S Z0T m • Z0L (1–m)Z0L Z0R Relay L 3R F Symmetrical Components Diagram for BG Line Fault R – + Lessons Learned Analyzing Transmission Faults | 59
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$12.00 Journal of Reliable Power Vo
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Journal of Reliable Power Volume 1
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Capable of running protection calcu
Page 7 and 8:
Both P and Q are two-input phase an
Page 9 and 10: TABLE 1: MHO ELEMENT POLARIZING CHO
Page 11 and 12: For an Aφ-ground fault on the syst
Page 13 and 14: The most onerous 3φ fault is one w
Page 15 and 16: If the angle is near +120°, then t
Page 17 and 18: Impedance-Based Fault Location Expe
Page 19 and 20: A. Simple Reactance Method From Fig
Page 21 and 22: Figure 9 shows a screen capture of
Page 23 and 24: The relay then performs “torque
Page 25 and 26: IX. LAB TEST SETUP Several line fau
Page 27 and 28: APPENDIX The following are the rela
Page 29 and 30: The following shows the analog math
Page 31 and 32: Digital Communications for Power Sy
Page 33 and 34: increasing security by a factor of
Page 35 and 36: The receiver amplifier is the major
Page 37 and 38: Noise from faults or other sources
Page 39 and 40: 80 60 40 20 R F 100 3 3 S 2 5 6 4 1
Page 41 and 42: 15. A POTT scheme implemented over
Page 43 and 44: Transmission Line Protection System
Page 45 and 46: One-Cycle Directional Element Figur
Page 47 and 48: element misoperation. Shunt reactor
Page 49 and 50: • SW2 is in Position 1 when there
Page 51 and 52: Complete reclosing relay supervisio
Page 53 and 54: IX. BIOGRAPHIES Armando Guzmán, P.
Page 55 and 56: 2 II. COMMON ZONE TIMING In June 20
Page 57 and 58: 4 ment 67N2 after a 1-cycle carrier
Page 59: 6 VI. ZONE 1 OVERREACH DUE TO CVT I
Page 63 and 64: 10 The phasors during the fault are
Page 65 and 66: 12 The prefault data from the backu
Page 67 and 68: 1 Adaptive Phase and Ground Quadril
Page 69 and 70: 3 C. Short Line Applications A shor
Page 71 and 72: 5 II. QUADRILATERAL DISTANCE ELEMEN
Page 73 and 74: 7 For the purpose of implementing t
Page 75 and 76: 9 The three-phase fault detection e
Page 77 and 78: 11 Zone 1 distance elements should
Page 79 and 80: 13 Fig. 24 shows the apparent resis
Page 81 and 82: 15 B. Adaptive Resistance Element F
Page 83 and 84: 17 Fig. 34 and Fig. 35 illustrate t
Page 85 and 86: Line Protection Bibliography Issue
Page 87 and 88: Hou, Daqing, and Jeff Roberts. “C
Page 90: ©2010 Schweitzer Engineering Labor
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