2016 Volume 136 Issue 12 Pages 918-926
The characteristics of creeping discharge propagating on the pressboard surface in vegetable-based oils (crude rapeseed oil and PFAE (palm fatty acid ester) oil) and commercial mineral oil with the thermally accelerated aging were investigated by applying AC high voltages of 60Hz to the needle electrode. The pressboard impregnated with the sample oil under the vacuum condition was immersed completely into the same oil, and the tungsten needle electrode was installed in the one side of pressboard surface without the counter electrode. The other side of pressboard has the grounded fine copper rod as a back side electrode (BSE). The discharge shape, streamer extension, streamer velocity, discharge current, discharge energy, pressboard surface tracking and pressboard puncture in the deteriorated oils were examined as a function of the voltage and its imposed time in comparison with those in the original oils. The AC creeping discharge in oils basically grows slowly in the distinctive shape which is characterized by many formations of a fine branching, flashing spot and current pulse under a fixed voltage regardless of the presence/absence of deterioration. However, in this paper, it is reported that the streamer length, streamer velocity, amplitude of discharge current pulses and discharge energy are significantly affected by the deterioration of oils.