Failure analysis of a 1500 kVA 13.2 kV/480 volt, delta-delta transformer

This paper discusses the events surrounding the catastrophic failure of a 1500 kVA, 13.2 kV/480 V, delta-delta cast-coil transformer. The transformer supplied a 480 V, three-phase, three-wire distribution switchboard serving HVAC and miscellaneous motor loads. Both utility feeders supplying the 15 kV switchboard suffered faulted operating conditions exterior to the building and were de-energized. The transformer subsequently failed after switching operations in the 15 kV switchboard supplying the transformer. The pages that follow examine the final condition of the transformer and enclosure, along with the 15 kV switchboard serving the transformer and the 480 V switchboard served by the transformer, to speculate how the transformer failed.