Transients Caused by Sequential Circuit Breaker Tripping Issued by Busbar Protection

Abstract

A study of transients in a high voltage substation 400/110 kV is presented in the paper. An analysis was carried out after a fault on the 110 kV busbar, which caused severe damage in the substation. Investigation was focused on a time frame of several sequential circuit breaker trippings. A first step of the study was collection of data from the primary and secondary system in the substation and the control centre. After numerous analyses of data an attempt was made to construct a precision model, which could be used in the computation. Appropriate models were developed for circuit breakers, voltage (potential) and current metering transformers, power transformers, surge arresters, overhead lines and an equivalent grid. The components of the power system can be modelled for the very particular purpose, which means that a different frequency model should be used and each element in this analysis has a specific frequency response. An attempt was made at very detailed modelling of a power transformer, air blast and SF6 circuit breakers. Computed results of fault currents were compared with measurements captured by the disturbance recorders in the field, mainly in differential numerical relays. Different switching schemes and different tripping sequences of several 110 kV circuit breakers were analysed with a constructed model in the millisecond range. Models of circuit breaker with different types of media, air blast and SF6 gas were used in the cases investigated. Modelling of the circuit breakers’ electrical arc was an important item in all cases in order to take into account the interaction between electrical arc and circuit current during the process of current interruption. The Schwarz/Avdonin equation is applied to model the dynamic behaviour of an electric arc. The fault studied was accompanied by a large short circuit current. For this particular case two types of circuit breaker, air blast and SF 6 were modelled. An important conclusion from those analyses was that sequential tripping of several circuit breakers does not cause superposition of overvoltages, because interruption the current happens when it is passing through the zero. Even the record from the substation and the disturbances recorder proves that each particular circuit breaker was successfully opened. On that basis, focus was put only on the final opening of the breaker and its arc extinction. The conclusion can be drawn that such a substation fault should have no influence on excessive overvoltages that can threaten the insulation of components in the substation.