Use of standard 87T differential protection on phase shifting transformers

Abstract

This paper describes universal, current based, differential protection for any three-phase, phase-shifting transformer having variable phase angle shift and variable turns ratio. The use of standard transformer differential protection for such applications was considered impossible in the currently applied protective relaying standards and practices. This universal differential protection method only requires stand-alone CTs and VTs on the two sides of the protected transformer. Thus, any buried current transformers within the tank of the protected transformer are not required regardless transformer construction details and internal on-load tap-changer configurations. At the same time, the actual position of any built-in on-load tap-changer is neither required as an input into the differential protection. The differential protection is self-adaptive. It automatically learns and adopts to the actual transformation ratio and phase-angle shift across the protected power transformer. Thus, any phase-shifting transformer regardless of its construction principles (symmetrical or asymmetrical) and design details (single-core, double-core or even of any complex design) can be entirely protected by using such differential protection.