Stochastic studies of overvoltages resulting from circuit breaker operation in industrial installations

Abstract

Switching overvoltages are stochastic in nature and the conditions which may result in serious overvoltages are numerous. It has been found that switching operation of vacuum circuit breakers in motor circuits can cause large transient overvoltages at the motor terminals or the transformer terminals. By predicting these transients and applying suitable overvoltage protection, the risk of failure to equipment can be reduced. In order to accurately predict system overvoltages, suitable simulation models are required which reflect the behaviour of the key components at power frequency as well as at frequencies corresponding to fast transients. In this research a stochastic vacuum circuit breaker model, a universal hybrid high-bandwidth (from power frequency to MHz) induction motor and a hybrid transformer model have been developed in the ATP version of the Electromagnetic Transients Program (EMTP). A large number of detailed studies were performed and the results indicate that some combinations of breaker characteristics, induction motor ratings and network topology, including system fault level, cable lengths, power factor correction capacitors and grounding practices may lead to severe reignition and voltage escalation problems. Consequently there is a need to establish, in general terms, which parameters, combinations of parameters and system configurations may lead to particularly severe transient conditions and to evaluate the performance of different protection measures. Rather than studying a large number of installations, a parametric sensitivity study was performed aimed at identifying the essential parameters that could lead to potential damage of the system equipment. Mitigating measures were also studied. The results of the parametric sensitivity studies and the effectiveness of different protection measures are presented. Two real case studies of industrial installations in Hong Kong were carried out with one where there was a failure of a transformer attributable to VCB switching, and one where VCBs were to be installed as replacements for oil circuit breakers. The results of the case studies compare well with predictions made in the simulations.