Analysis of partial discharges detected in 420kV OIP bushings

The 2015 CIGRE Transformer Reliability Survey reveals that bushing failures accounted for over 30% of reactor failures from 2000 to 2010. Of the reported failures, those that originated in bushings often lead to severe consequences with almost 50% of the failures leading to fires or explosions. [1] Such was the case with a large utility operating a 400kV network. After experiencing a series of violent bushing failures, the utility embarked on a program to perform additional offline power factor and capacitance measurements in hopes of preventing additional failures. The bushings of concern were oil impregnated paper (OIP) bushings of a certain make and vintage which were installed on auto-transformers and reactors. Investigations into the root cause of the bushing failures were inconclusive. Since the offline power factor and capacitance measurements were not always successful in detecting bushings that were in a degraded state, the utility decided to investigate alternative methods to identify failing bushings.