THE ELECTRICAL BREAKDOWN CHARACTERISTICS OF OIL-PAPER INSULATION UNDER STEEP-FRONT IMPULSE VOLTAGES

Disconnecting switch operations in gas insulated equipment cause trans ient voltages with risetimes as steep as 5 to 20 nanoseconds and magni tudes as high as 2.5 pu. There is very little information on the effec t of these transients on oil-paper insulated equipment. There have bee n reports, however of transformer and bushing failures caused by these transients. The electrical breakdown characteristics of oil-paper ins ulation under steep front impulse were studied in this project, which was co-sponsored by the Canadian Electrical Association and B.C. Hydro . V50 (50% breakdown probability voltage) breakdown data was obtained with steep front (10 ns/2500 mus), lightning and switching impulse wav eforms. Insulation breakdown voltage vs breakdown time (V-t) data and multiple impulse breakdown data were obtained with the steep front imp ulse waveform. The V50 results showed that the breakdown strengths wer e lower for steep front impulses than for lightning impulses. The mult iple impulse breakdown results showed that oil-paper insulation breakd own strength can be lower than 100 kV/mm. These results are alarming, since they suggest that oil-paper insulated equipment subjected to ste ep front transients will fail at voltages below the lightning impulse design level (BIL). The Volt-time data had a discontinuity. The breakd own process at risetimes below about 50 ns was different from the brea kdown process at risetimes above 50 ns.