The Ultra-Saturation Phenomenon of Loaded Transformer Energization and Its Impacts on Differential Protection

Abstract

A sort of unusual maloperation of the differential protection of the transformer and delayed maloperations due to the loaded transformer energization are reported. A second-order equivalent circuit-based model is set up to investigate this phenomenon. With rational assumptions and simplifications, the analytical solution of the main flux of the transformer is obtained. On the basis of the expression of the flux, the delayed maloperation can be explained in a rational way. Furthermore, a peculiar transient phenomenon of transformer energization, specified as in this paper, is revealed. It was substantiated to be a more severely adverse condition in terms of the security of the transformer differential protection. In the extreme case, all existing blocking schemes implemented with the identification of the characteristics of inrushes or fluxes will be of no effect regardless of how to set the threshold. A variety of the simulation tests proves that the delayed maloperation of the differential protection and ultra-saturation of the transformer core may occur in the occasion of loaded transformer energization.