Creeping Discharge Characteristics over Aged Rapeseed and Mineral oils / Pressboard Interface under AC Divergent Field

The behaviour of creeping discharge along the oil/pressboard interface in heat-accelerated aging oils; rapeseed oil and mineral oil, was investigated by applying to the needle electrode variable ac voltages up to 40 kV (effective values) with 60 Hz. The clean pressboard impregnated with sample oil under the vacuum condition was immersed completely into the same sample 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 were examined in comparison with those in new oils without the…