Differential Protection Methodology for Arbitrary Three-Phase Power Transformers

This paper describes how to provide universal, current based, differential protection for any three-phase power transformer, including phase-shifting transformers with variable phase angle shift and special converter transformers with nonstandard but fixed phase angle shift (e.g. 22.5deg). The use of standard transformer differential protection for such applications is considered impossible in the protective relaying standards and practices currently applied. This universal differential protection method only requires standalone CTs on all sides of the protected transformer. Thus, any buried current transformers within the tank of the protected power transformer are not required regardless transformer construction details and internal on-load tap-changer configurations.