Transformer differential protection with phase angle difference based inrush restraint

Abstract

A new technique for blocking the operation of the transformer differential relay when subjected to magnetizing inrush current is presented in this paper. The relay differential and restraint currents are calculated first, and the fundamental-frequency components of the two currents are then compared to identify the phase angle difference (PAD) between the corresponding transformer primary and secondary currents. Distinguishing the magnetizing inrush current from an internal fault current is accomplished by the presence of more than 90° phase shift between the two currents during internal faults, and the absence of this in the case of magnetizing inrush and unfaulted transformer currents. This technique is investigated using an EMTP-RV simulation based model of a typical 3-phase 2-winding power transformer embedded in a power system fed from both ends. Results have confirmed that the PAD principle is capable of categorizing transformer operating modes within a cycle of the power system frequency. Also, the relaying decision is unaffected by current transformer (CT) saturation effects.