Protection of standard-delta phase shifting transformer using terminal currents and voltages

• A new terminal voltage–current based protection is proposed for standard-delta PST.
• The proposed voltage–current relations represent the normal operating condition of the PST.
• Normal operating condition of the PST is monitored to detect internal/external faults.
• The proposed method offers more stable/secure solution than the differential relays.

This paper presents a new standard-delta phase shifting transformer (PST) protection technique based on the normal operating voltage–current (v–i) relationships of the PST equivalent circuit. These voltage–current relations are used to define the v–i differential equations, which are further used to detect and discriminate the internal and external faults. Using the v–i differential relationships, the proposed protection relay computes small or zero (ideally) differential quantity during normal and external fault conditions. However, in the event of an internal fault, the v–i relationships become invalid and the proposed differential relay therefore computes significantly large differential quantity. A comparative performance analysis of the proposed protection technique and the former methods, that make use of the current differential principle, suggest that the proposed technique offers a more secure protection solution of the standard-delta PST. It successfully detects and discriminates the internal/external faults, while remaining stable during a magnetizing inrush current and the saturation of the series winding, and offers a high degree of tolerance against external faults with current transformer saturation. A real representation of a standard-delta PST has been modeled in PSCAD/EMTDC to simulate various normal and faulted conditions to test and validate the performance of the proposed protection technique. Implementation of the proposed protection algorithm requires currents, voltages and tracking of the tap-changer position.