Asymmetry Characteristics Of Prpgressive Short-Circuits On Large Synchronous Generators

Short-circuit tests have shown that, under conditions of maximum asymmetry, three-phase short circuits near generator terminals may have one or possibly two phases in which the current does not reach zero for several cycles. Particularly in the application of high- speed high-voltage circuit breakers, there could be a serious question that this phenomenon might prevent normal circuit opening of such circuit breakers, with possible damage to the circuit breaker and the system. This investigation was undertaken to develop a method for calculating such conditions that would agree with the tests and could be used for examining future applications. The method developed takes into account that non-simultaneous closure of the three phases can substantially extend the time to reach current zero, and produced good agreement with the tests previously made. When applied to a major generating station addition now under construction, it was found that an improbable but possible sequence of phase closures could produce a continuing short circuit where one phase did not reach zero for about eleven cycles. However, when the phases which did reach zero were allowed to open at their normal opening times for a 2-cycle circuit breaker, the third phase reached zero in three to four cycles.