Aj. Vandermaar et al., THE ELECTRICAL BREAKDOWN CHARACTERISTICS OF OIL-PAPER INSULATION UNDER STEEP-FRONT IMPULSE VOLTAGES, IEEE transactions on power delivery, 9(4), 1994, pp. 1926-1935
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.