Studies on the dielectric strength of oil-polymer composite insulation under variable frequency AC voltages

Breakdown characteristics of silicone oil-polyethylene composite insulation under ac voltages at different frequencies were studied using a needle-plane electrode system. Higher breakdown voltages have been observed with LDPE film as a barrier than those with oil alone for frequencies below power frequency. The breakdown voltage decreased with increased frequency with film, whereas, with oil alone, no such frequency effect was observed for the range of frequency considered (2 Hz to 1 kHz). At low frequencies the breakdown always occurred on positive polarity, although pd patterns indicate the occurrence of a large number of discharges in the negative half cycle.