Analysis of Bus
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1 Analysis of Bus Tie Circuit Breaker Tripping in Medium Voltage Side Caused by Transformer Fault in Low Voltage Side FENGYI1 WANG Shaojie1 TIAN Wenping1 WANG Yajie2 (1. Wuzhong Power Supply Bureau, Ningxia Grid Corporation, Wuzhong 751100, China; 2. School of Electric Engineering, Xi'an Jiaotong University, Xi'an 710049, China )
Abstract: The impact on operation backup protection in medium voltage side
caused by phase to phase short circuit fault in low voltage 10KV bus side of 110KV transformer is analysed. Then, corresponding solutions such as fitting backup protection time, changing operation mode of system, etc, are proposed for reducing influence of low voltage side phase to phase fault on backup protection in medium voltage side. Through failure model simulation and theoretical calculation, the feasibility of previous solutions is verified, which may offer a reference for the setting calculation of power system.
Key words: transformer; backup protection; bus tie circuit breaker; parallel
operation; setting calculation 0 Introduction In April 6, 2011,the low voltage side of a 110kv substation main transformer T1 10KV bus carried by the line A-phase intermittent arc ground, making the bus electromagnetic voltage transformer (TV) excitation current increases suddenly saturated, resulting insevere ferroresonance over-voltage, causing the TV cabinet and white discharge breakdown, the occurrence of arc-phase short circuit fault, the TV explosion burned. When the backup complex voltage over-current protection in low voltage of main transformer T1 trip off the main transformer low-voltage side of the circuit breaker 501 after tripped off 10KV bus tie circuit breaker 500,the backup complex voltage over-current protection in medium voltage side of the main transformer T1 and T2 trip off 35KV bus tie circuit breaker 300 at the same time.According to field failure status of the investigation and the experience of operation and maintenance,The 35KV bus side and circuit in medium voltage side of main transformer is not the failure, the backup protection of the main transformer should not tripped off the bus tie circuit breaker 300. Whether the protection action is a malfunction, this paper resolves the doubts in protection action.
1 operation mode before short-circuit fault and field condition when
it is fault operation mode in substation before short-circuit fault shown in Figure 1. 2
Fig.1 main wiring scheme of operation mode in substation before short-circuit fault 110kv incoming lineⅠbreaker 111 run, incoming lineⅡbreaker 112 hot reserve, main transformer T1 and T2 run; breaker 301and 302 and the bus tie circuit breaker 300 run, 35kv system have not external power; breaker 501 run, breaker 500 and the bus tie circuit breaker hot reserve; 10kvⅠbus TV run andⅡbus TV cold reserve. field protection status as follow after short-circuit fault: the backup protection device panel in low voltage of main transformer T1 trip off indication red light, when reach 1516ms complex voltage over-current protection Ⅰrun and trip off bus tie circuit breaker 500 (actually in the hot standby); when reach 1800ms complex voltage over-current protectionⅡ run and trip off breaker 501 in low voltage of main transformer, 10kv system voltage trip indication of the back-up protection device of medium voltage side of the panel in the main transformer T1 red light, when reach 1816ms complex voltage over-current protectionⅠtrip the bus tie circuit breaker 300 of medium voltage side; the backup protection device panel in medium voltage of main transformer T1 trip off indication red light, when reach 1800ms complex voltage over-current protection Ⅰrun and trip off bus tie circuit breaker 300.
2 Reason Analysis of Skip Action the bus tie circuit breaker 300 action of the medium voltage side of tripped, protection malfunction possibility is the existence of further analysis, the need for protection action. 2.1 Flow Directions of Fault Current and Analysis of Protection Action The main transformer T1 and T2 high voltage side of the parallel operation, the 3
low pressure side of the TV after the explosion caused the low voltage side of the bus phase short circuit fault current is divided into 2-way flow to the point of failure, shown in Figure 2.
Fig.2 Flow diagram of 1II and 2II when low voltage side phase to phase fault occurs 1) Fault current1IIspreads to the low voltage side to the point of failure after passing through high-voltage side of the main transformer T1; the backup protection in low voltage of main transformer T1 run, when reach 1516ms complex voltage over-current protection Ⅰrun and execute the command of tripping off bus tie circuit breaker 500(actually the bus tie circuit breaker in the hot standby, in quartile); when reach 1800ms complex voltage over-current protectionⅡ run and trip off breaker 501.The combination of value analysis of fault current protection , the protection action is correct.