Influence of CT Y-delta Transform on Measure of Sympathetic Inrush

The paper is concerned with the effect of CT Y- delta transform on measure of sympathetic inrush in order to discuss whether the existing inrush current identification algorithms are applicable to sympathetic inrush. According to the characteristics of sympathetic inrush, the effect of CT Y-delta transform on second harmonic proportion of differential current is investigated in the case of the direction of two phases sympathetic inrush deviated from time axis is same or not. Theory analysis and simulation show that second harmonic proportion of differential current may be less than that of each phase inrush current while direction of two phase sympathetic inrush is reverse. Thus, second harmonic proportion may decrease due to CT Y-delta transform, and transformer differential relays may mal-operate in this condition. It is believed that the study will provide a theoretical basis to discover the inherent reason for mal-operation of transformer differential relays caused by sympathetic inrush.