High voltage transformer bushing problems

Bushing is an important part of power transformer. It is used to insulate the incoming or outgoing conductor into or out of a grounded barrier, in power transformer case is the transformer main tank. The bushings connect the windings of the transformer to the supply line and insulate the feed through conductor from the transformer main tank. From the physical and electrical point of view, bushing appears to be the weakest part of the transformer. The starting of the bushing breakdown process could not be detected easily and could lead to the sudden failure of the bushing, even the failure of the whole transformer. Currently, any time-based-maintenance conducted in PLN P3B Jawa Bali based on manufacturer's manual had been proved not sufficient to recognize any degradation in the bushing. Sometimes it is very hard to find the real root cause of the failure, and getting only the suspected cause of failures. This paper presents some field experiences concerning transformer's bushing failures, along with some facts and findings related to the failure.