Relay Protection of Generator Station Service Transformers

Generator station service transformers, also called unit auxiliary transformers, are subjected to very rigorous short circuit duty. These faults fall generally into three areas: Faults within the transformers themselves, low-side faults between the transformer and the low-side bank breakers, and faults on the station service supply buses. The purpose of this paper is to show how station service transformers should be protected, and why any protective scheme can only be effective if these transformers are properly specified for UAT service.