Comparison of dielectric strength of transformer oil at DC and multimillisecond pulses

The goal of this work was determining dielectric strength of transformer oil at repetitive application of multimillisecond pulses as compared to DC voltage. A sphere-to-plane test gap was varied from zero to 15 mm to mimic field nonuniformity from unity to about four as shown by field analysis. Both clean and contaminated oils were tested. Standard ASTM D 1816- 84a breakdown tests preceded DC and pulsed testing. The latter was conducted at up to 160 kV at pulse repetition rate (PRR) up to 120 Hz, with 1-ms risetime and much longer load-dependent fall time. The main results are as follows. In conformance with published data, the breakdown voltage, V_br, is higher at negative polarity for both fresh and contaminated oil, and the results´ spread is smaller. However, there is a marked difference between pulsed and DC breakdown at both polarities for soot-contaminated oil: pulsed V_br is higher than its DC counterpart by 10% to 20%, on the average, depending on PRR and polarity. Most probably, the bridge mechanism governs breakdown of contaminated oil.