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The Analysis of Mal-operation of Protection Relay for Detecting Disconnection of Neutral Grounding Reactor Yeonwook Kang, Joosik Kwak, Heeyoung Ryu, Dongjin Kweon and Eungbo Shim Korea Electric Power Research Institute 103-16 Munji-dong, Yuseong-ku, Deajeon, Korea Abstract As the electric system is getting larger to meet the increasing demand for electric power, the rating of power apparatus is becoming inevitably higher in its working voltage and larger in its capacity. According to KEPCO reports, power transformers in the KEPCO system have undergone troubles such as winding short insulation breakdowns every year since 1981. The cause of these troubles was high one line grounding fault currents in KEPCO systems that had direct grounding systems. KEPCO has installed the NGR(neutral grounding reactor) to lower this fault current and reduced winding short insulation breakdowns in power transformers. But when a circuit breaker opened a no load bus, some trips of circuit breakers to protect transformer have occurred due to an incorrect operation of the 59GT(overvoltage ground relay) that detects disconnections of NGR. Therefore, in this paper, we analyzed the cause and examined the effect of a time delay circuit to prevent wrong operation of 59GT. Keywords: NGR(Neutral Grounding Reactor), EMTP, Time Constant, Transformer mal-operation of the 59GT relay by EMTP(Electro- 1 Introduction High kV magnetic transient program). And we also examined transformers when one line-to-ground fault occurred in the effect of the time delay circuit to prevent incorrect KEPCO's distribution line because the system was a operation of the 59GT. direct fault currents grounding flowed system. In into order 154/23 to reduce transformer faults such as winding short insulation, the 2 Background NGR was installed between the transformer neutral There has been a large increase in power transformer point and the ground. problems in the KEPCO system since 1980. According If NGR is disconnected and one line-to-ground fault to the KEPCO survey, 92 transformers (154kV/23kV) occurs, then there are line-to-line voltages on have experienced problems from 1981 to 1988 and 62 un-faulted lines that cause insulation problems on of those were damaged by fault currents that flowed utilities and loads. through neutral points of transformers. Therefore, the 59GT has been used to watch for To reduce the fault current flowing into the disconnection of NGR in KEPCO but some trips of transformer under the condition of a fault, the KEPCO circuit breaker to protect transformer have occurred changed the 23kV side of transformer from a direct through the improper operation of the 59GT, when grounded system to a neutral grounded system using a circuit breaker opened only no load and line charged reactor as shown in Fig 1. bus. The value of the reactor L is determined to make one In this paper, we introduced the background of use of phase-to-ground fault current equal to 85% that of a NGR in KEPCO system and analyzed the cause of the three phase-to-ground fault current like equation (1). The value is about 6 ohms. L= 13.2kV/63.5V). The input of 59GT is 2X1 − X 0 − X 2 3 → ---------------(1) → → → EPT = E A + EB + EC ---------------(2) where X0 : Zero Sequence Impedance where EPT : Input Voltage of 59GT X1 : Positive Sequence Impedance 63.5V 13.2kV 63.5V EB : Voltage of Phase B of 22.9kV bus x 13.2kV 63.5V EC : Voltage of Phase C of 22.9kV bus x 13.2kV X2 : Negative Sequence Impedance E A : Voltage of Phase A of 22.9kV bus x VB N VC VA L IL IF IA IC IB Under normal conditions, the input voltage of the XC XC XC 59GT, EPT is near zero as shown in Fig 3, because the voltages of each phase of 22.9 kV bus are nearly the Fig 1. NGR installation to reduce the fault current same and their vector sum is near zero. 3 Detection of NGR disconnection B To reduce this high failure rate(approx. 3%), KEPCO EB E A = 63.5 V E B = 63.5 V EC = 63.5 V has inserted the reactor between the neutral point of the 23kV side of transformer and the ground. Accordingly, faults in transformers have decreased N since 1993. E PT = 0 V If NGR is open and one phase-to-ground fault occurs, then the voltages of the unfaulted phases are increased A EA by 1.73. Therefore KEPCO has used the 59GT to open C EC (a) Phasor diagram the circuit breakers of the 1st & 2nd side of transformer and to protect the loads from overvoltages on the unfaulted phases when the one phase-to-ground fault occurs under the condition of the disconnection of NGR, as shown in Fig 2. 300 [V] 200 100 0 22.9kV BUS 154kV BUS A 1차 CB M .Tr 2차 CB B : CB Close -100 : CB Open -200 ∆ Y NGR -300 0 10 (file PAPER_04.pl4; x-var t) v:PT_A -PT_G 20 30 v:PT_A -PT_B v:PT_B -PT_C 40 [ms] v:PT_C -PT_G 50 (b) Simulation result by EMTP Fig 3. Voltages at unfaulted condition 59GT 59GT If phase A is faulted to ground, the ground point Fig 2. Detection of disconnection of NGR by 59GT moves to N' from N and voltage of phase A becomes 59GT detects the unbalanced voltage of 22.9 kV bus zero and voltages to ground in phases B and C through the PT(potential transformer, Y/open delta, increase. Therefore EPT does not become zero, but about 106 V as a result of a simulation under the assumption that the ground fault resistance is ignored as shown in Fig 4. 300 [V] 200 100 0 EB B EB = 78 V -100 EPT EC = 78 V EPT = EB + EC = 106 V -200 -300 0 10 (file PAPER_06.pl4; x-var t) v:PT_A -PT_G N 20 30 v:PT_A -PT_B v:PT_B -PT_C 40 [ms] v:PT_C -PT_G 50 (b) Simulation result by EMTP N' Fig 5. Voltages at one phase-to-ground fault A in disconnection state of NGR C EC In one phase-to-ground fault, as we know, the input (a) Phasor diagram voltage of the 59GT is about 106 V in the connected state of NGR and 190 V in the disconnected state of NGR. The 190 V value is based on the assumption that 300 [V] the ground fault resistance is ignored in one 200 phase-to-ground fault and NGR is open. If the ground 100 fault resistance is higher than zero, the voltage is lower 0 than 190 V. According to a KEPCO report, it is assumed that the -100 maximum ground fault resistance is 50 ohm in -200 KEPCO’s system, and that the minimum operating -300 0 10 20 (file PAPER_05.pl4; x-var t) v:PT_A -PT_G 30 v:PT_A -PT_B 40 v:PT_B -PT_C [ms] v:PT_C -PT_G 50 voltage of the 59GT is 120 V. Therefore when the (b) Simulation result by EMTP input voltage of the 59GT is above 120 V, the Fig 4. Voltage at one phase-to-ground fault transformer is tripped to prevent the overvoltage of the system and load since one phase-to-ground fault may If phase A is faulted to ground in the disconnection occur in the disconnection state of NGR. state of NGR, the ground point moves to N'(A) from N like an ungrounded system, and voltages to ground in 4 Mal-operation of the 59GT phases B and C increase in magnitude by 1.73 times as KEPCO operates two buses on the second side of shown in Fig 5. 154/23 kV transformers with section breakers for the purpose of parallel load shedding. For regular E BA E PT B provides power to all loads and the other charged bus E B = 109 V EC = 109 V EPT = E B + EC N = 190.5 V C A N' examination of circuit breakers, when one bus ECA (a) Phasor diagram is open as shown in Fig 7, the 1st & 2nd breakers of the transformer sometimes are tripped by the mal-operation of the 59GT relay. After mal-operation of the 59GT relay, input voltages of the 59GT were measured in the same conditions as a section breaker opened charged bus without load as shown in Fig 6. But the voltages were below 120 V for the operation of the 59GT . 80 [V] 60 40 20 0 -20 -40 0.00 0.02 0.04 0.06 0.08 [s] 0.10 [s] 0.10 [s] 0.10 (file PAPER_C3_5_2.pl4; x-var t) v:XX0070-E_PT (a) Input voltage of 59GT in M substation (a) Discrepancy time = 2.8msec 120 [V] 80 40 0 -40 -80 (b) Input voltage of 59GT in Y substation -120 0.00 0.02 0.04 0.06 0.08 (file PAPER_C3_5_3.pl4; x-var t) v:XX0070-E_PT Fig 6. Overvoltages at opening no load bus (b) Discrepancy time = 10msec 150 [V] When a circuit breaker is opened and closed, it has a 100 discrepancy time among phases as shown in Table 1. 50 The input voltage of the 59GT appears different 0 according to the discrepancy time of the circuit breaker. -50 For the analysis of the mal-operation of the 59GT relay, -100 it is assumed that the discrepancy time of the circuit -150 breaker may be longer than the standard time. Because the section breaker open for the regular examination -200 0.00 0.02 0.04 0.06 0.08 (file PAPER_C3_5_4.pl4; x-var t) v:XX0070-E_PT after several years, the section breaker remains closed (c) Discrepancy time = 20msec for a long time in a state of current flow that may Fig 7. Input voltages of 59GT by discrepancy times make the contacts of circuit breaker stronger due to When the discrepancy time is 2.8msec, the input Joule's heat. voltage of the 59GT becomes about 50V. However the Table 1. Discrepancy time (Below 72.5 kV, KEPCO) discrepancy time becomes longer than 2.8msec, and Classify Time the input voltage of the 59GT is higher. As shown in 3 phase open discrepancy time Below 1/6 cycle Fig 7, when the discrepancy time is 20msec, the 3 phase close discrepancy time Below 1/4 cycle voltage becomes greater than 180V that can operate the 59GT relay sufficiently. In this paper, it is assumed that discrepancy times of section breakers are 2.8, 10, 20msec for the simulation 5 The improved circuit of the 59GT relay of input voltage of the 59GT by EMTP, when a section The transformer was tripped several times through the breaker opens a charged bus without load. The result is mal-operation of the 59GT relay, when a section shown in Fig 7. breaker opened a charged bus without load. Therefore the 59GT relay should be improved in order to prevent mal-operation. The improved circuit of the 59GT relay Ground is shown in Fig 8. The input voltage of the 59GT is Transactions on Industry Applications, vol. 36, rectified by diodes and then the time for operating pp.16-21, 2000. excitation coil is delayed by the R-C time constant  Garry E. Paulson, "Monitoring Neutral-Grounding until the voltage of the coil becomes 24V. It is helpful Resistors ", IEEE, 1999. to prevent mal-operation within about 2 cycles as  John P. Nelson and Pankaj K. Sen, "High shown in Fig 9. Resistance Grounding of Low Voltage Systems: A Differential Protection Relays", IEEE Standard for the Petroleum and Chemical Industry", IEEE, 1996. 59GT R1 R2 V (t ) Biographies R C VIN Fig 8. Improved circuit of 59GT 150 [V] 100 50 0 -50 -100 -150 -200 0.00 0.02 0.04 0.06 (file Paper_C3_5_4.pl4; x-var t) v:XX0070-E_PT 0.08 [s] 0.10 v:RY_T -RY_G Fig 9. The operating voltage of 59GT relay at abnormal condition 6 Conclusion In this paper, we analyzed the cause of mal-operation of the 59GT relay and appropriateness of the improved circuit of the 59GT relay. When the circuit breaker has not done operation with closed state for a long time, the discrepancy time of it may become longer than the standard time. And it sometimes causes the mal-operation of the 59GT relay to trip the 1st & 2nd circuit breakers of transformers. To prevent this mal-operation, 59GT relay should be improved. In this paper, we analyzed that the improved 59GT is adequate to prevent it by EMTP simulation. REFERENCES  "A study for the reduction of the power transformer failures", KEPRI Final Report, 1992.  Peter E. Sutherland, "Application of transformer Yeon wook Kang : He received B.S. and M.S. degree from Chungnam University, Korea in 1988 and 1990, respectively. He has been worked for Korea Electric Power Research Institute since 1993. His research interests include lightning outage rate calculation and insulation coordination. Joo sik Kwak : He received B.S. and M.S. degree from Chungbuk University, Korea in 1994 and 1996, respectively. He has been worked for Korea Electric Power Research Institute since 1996. He is interested in the field of analysis of power system overvoltages and lightning protection. Hee young Ryu : He received B.S. from Dankook University, Korea in 1995. He has been worked for Korea Electric Power Research Institute since 1995. His research interests include insulation coordination and substation protection. Dongjin Kweon : He received his BSEE from Seoul National Industry University in 1986. And he received his MSEE and Ph. Doctor from Soong-sil University in 1995. He has been worked for Korea Electric Power Research Institute since 1995. He is interested in the field of diagnostic techniques and maintenance for power equipment. Eung bo Shim : He received B.S. degree from Hanyang University, Korea in 1982. He has been worked for Korea Electric Power Corporation since 1982. He is working at Korea Electric Power Research Institute. His research interests include the analysis of power system overvoltages and insulation design.