VFT V-T curves for SF6 and SF6:N2=1:1 in a 180/110 mm and coaxial duct at pressures of 1 and 2 bar with small values of surface roughness

The performance of SF₆ and SF₆/N₂=1:1 in a 180/110 coaxial gap under disconnector generated very fast transients (VFTs) is investigated. Gas pressures of 1 and 2 bar were used and the effects of small values of high voltage bus surface roughness were investigated by using buses with maximum protrusions of 10 μm (smooth) and 40 μm (rough). Breakdown prediction was done using the streamer criterion and it was found to be inaccurate above pressures of 1 bar. Very fast breakdown was recorded which indicated that the total time lag before breakdown could be as short as 12 ns. It was found that the addition of N₂ to SF ₆ had a negative effect on the withstand capability of the gas mixture at times to breakdown of less than 500 ns. It was also found that small values of surface roughness have a far smaller effect under high frequency conditions than expected. Future work should seek to confirm these findings by expanding the study to include wider ranges of gas mixture, pressure and surface roughness