Turbogenerator short circuits with delayed current zeros

It has been observed in practice that, following a fault at the terminals of a synchronous generator, the armature currents may not pass through zero for some considerable time, and this phenomenon may result in serious damage to a generator breaker attempting to interrrupt the fault current. In this paper, the various factors which affect the non-zero time are investigated. An analytical study is made of the 3-phase fault, to illustrate the importance of the various machine parameters and the prefault-load conditions. Unbalanced faults are considered with the aid of a digitial-computer solution of the phase-variable equations of the machine. Using this phase model, the study is extended to include a delta-star connected generator transformer, and the effects of sequential interruption of the fault currents and the arc in the generator breaker are also investigated. In particular, it is demonstrated that the redistribution of current following the interruption of one phase will considerably reduce the non-zero time in the phase worst affected, and that the breaker arc, by inducing earlier current zeros, also has a major mitigating effect on the non-zero problem.