Commutation process in gas circuit breakers: close-operation and commutation failure during an open-operation

Exemplary for gas circuit breakers, a generator circuit breaker was chosen for this investigation, because these breakers are well suited to very high currents and have therefore severe conditions for the commutation process. A generator circuit breaker is designed as an operational switch to simplify the processes in a power plant and under service conditions to carry the nominal current. However, under fault conditions the generator circuit breaker is able to close into short-circuit and to interrupt this current. To fulfill this task the current has to commutate from a nominal contact system to the alternate current path (arcing contact system). Due to safety reasons, the crucial task of current commutation needs to be performed with high reliability. In This work, current commutation studies of a generator circuit breaker have been carried out. After a brief introduction of theoretical current commutation models that are valid under open, close and failure operating conditions in circuit breakers, predictions of these models are compared with experimentally observed commutation currents. In particular the effects of breaker resistance and inductance on commutation time are analyzed theoretically. The commutation currents are measured in a principle test device. It is found that the arcing path resistance is the decisive parameter affecting commutation time during an open operation while for close operation, both arcing path resistance and inductance play an important role. The good agreement between the theoretical model and the experimental current commutation data provides insight into the successful commutation processes in generator circuit breakers. The commutation processes in circuit breaker mainly depend on the external current that flows through the breaker, the current asymmetry, the breaker´s parameters and the phase angle of the applied current at which the contacts are opening or closing. From an analysis of breaker´s parameters, it is found that by reducing the arcing path resistance, a longer breaker lifetime can be ensured. On the basis of the results presented here, design criteria for gas circuit breakers can be deduced.