Breakdown Characteristics of a Transformer Oil in Moderately-Uniform Fields under Lightning Impulse

This paper presents an experimental study on the breakdown characteristics of a mineral transformer oil in moderately-uniform fields under standard lightning impulse. To achieve the moderately-uniform fields with the field factors from 5.5 to 31.3, the rod to plane electrode geometry was applied with the rod tip radius of 0.5 mm and gap distances from 3 mm to 25 mm. The negative breakdown voltages raise up steeply against the gap distance when the gap distance is lower than 10 mm and for higher gap distance the increasing rate of the breakdown voltage is reduced. Based on the simulated maximum electric field under the breakdown voltage, the negative transition field factor boundary between streamer initiation dominated breakdown and streamer propagation dominated breakdown is deduced at 9.6 which experimentally verifies the previous extrapolation based estimation. The positive breakdown voltages were also measured under the same conditions, which indicated that all the breakdowns are dominated by the streamer initiation in such moderately-uniform fields. This also evidences that the transition field factor boundary is polarity dependent and negative lightning impulse shows a lower transition field factor boundary value compared to the positive one.