Application of Laplace transform to analyse delayed current zeros in power stations which employ three-winding step-up transformers

The present paper investigates the conditions which make possible the occurrence of delayed current zeros in power stations equipped with three-winding step-up transformers. The current resulting from a fault at the LV-terminals of the three-winding step-up transformer is usually characterized by a high degree of asymmetry and can lead to delayed zeros. Despite the delayed current zeros resulting from this type of fault are not considered in IEEE Standard C37.013-1997 (R2008) and in its amendment IEEE Standard C37.013a-2007, they should be taken into account when studying the suitability of a generator circuit-breaker for the application. The conditions which make possible the occurrence of such a phenomenon are analysed by means of Laplace transform. The corresponding results in time-domain are validated by comparing the waveform of the current obtained with ATP simulations.