Pre-breakdown characteristics of contaminated power transformer oil

In this paper we have studied pre-breakdown characteristics of transformer oil in the presence of different levels of contamination. The contaminant is fibrous dust from pressboard insulation used for high voltage transformers. The contamination level investigated in the present study ranges from 0.0025 to 0.0075% by weight. The conduction current under dc voltages were recorded and bridging effect was monitored using optical images. It has been found that the conduction current typically increases with the applied voltage, the duration of the voltage application and the level of contamination. Optical images revealed that bridging occurs when oil is subjected to the voltage and bridge dimension increases with the contamination level and the duration. The current flowing between the two spherical electrodes is closely related to the bridging dynamics.