Forensic Examination and Failure Analysis of a 220-MVA Step-Up Transformer Fire

The paper presents the results of investigation and engineering analysis by Exponent of the failure of a 220-MVA generator step-up transformer in Southern California. The incident was caused by a dielectric breakdown from the high-side C-phase bushing to the side wall of the transformer. The ensuing fire continued for 27 hrs, fueled by approximately 11,000 gal of dielectric HYTRANS 61 oil. The paper will discuss the investigation conducted as per the IEEE C57.125 “Guide for Failure Investigation, Documentation, and Analysis for Power Transformers and Shunt Reactors,” [1] and the engineering review and analysis to identify the likely failure scenario(s).