Transformer differential protection with phase angle difference based inrush restraint

• A new technique for blocking the operation of the transformer differential relay when subjected to magnetizing inrush current is presented.
• The technique uses the phase shift between the 60 Hz components of the respective transformer primary and secondary currents as a restraint signal.
• With the integration of this technique, the transformer differential relay performance has been substantially improved in terms of reliability and immunity to CT saturation.
• The implementation of the presented scheme does not require complex computation and can be easily incorporated into existing digital differential relays.
• Discriminating internal fault currents from another type of disturbances in power transformers is efficiently performed within at most one cycle of the power system frequency.

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.