Investigation of 150/66 kV 100 MVA Transformer Internal Failure

An investigation was performed upon the failure of a 150/66 kV 100 MVA power transformer and nearby 66 kV arrester during heavy rain. To verify the possible cause of the failure, an evaluation was conducted by performing visual inspection on the transformer and the arrester after breakdown, analyzing related historical assessment and maintenance data, evaluating recorded lightning activity data from lightning detection system and generating relevant simulation using transformer nameplate and actual configuration. Majority of the historical assessment and maintenance data results showed no indication of transformer early breakdown, but the sweep frequency response analysis (SFRA) test result showed severe deformation on primary and secondary winding. According to the simulation using 20 kA 1.2/$50~\mu $ s lightning strike on 66 kV transmission line connected to the transformer, overvoltage on the transformer reached 1.69 p.u. for primary winding and 8.47 p.u for secondary winding. However, according to recorded lightning data during time of failure, there's no nearby lightning strike around the transformer or the transmission lines connected to it. Moreover, lightning overvoltage would initially shatter the transformer's bushing rather than directly deforming the transformer tank. The simulation result for short circuit between primary winding phase R to ground showed identical current and voltage waveform with digital fault recorder data during breakdown. According to data and simulation result analysis, lightning activity has no contribution toward the failure of the 150/66 kV 100 MVA transformer. The possible cause of transformer breakdown was internal failure in form of arcing between primary winding phase R to ground.