Influence of trace SF<inf>6</inf> on discharge characteristics of SF<inf>6</inf>/N<inf>2</inf> gas mixture under lightning impulse

Abstract

As one kind of synergistic effect gas, the discharge characteristics of SF 6 /N 2 gas mixtures would be influenced significantly by trace amounts of SF 6 . That is, the discharge characteristics of pure N2 is quite different from that of SF 6 /N 2 gas mixtures with relatively low content of SF 6 . For the application of SF 6 /N 2 gas mixtures in power system equipment, this paper studied on the discharge characteristics of SF 6 /N 2 gas mixtures with low SF 6 mixing ratio in extremely non-uniform electric field under positive and negative lightning impulse. Meanwhile, the characteristic of SF 6 /N 2 gas mixtures were compared with that of pure N 2 and SF 6 . The experimental results indicate that the 50% breakdown voltage increases linearly with the rise of gas pressure under negative lightning impulse, and the rising rate of breakdown voltage with pure N2 is greater than SF 6 /N 2 gas mixtures even greater than pure SF 6 gas. At high gas pressure, the 50% breakdown voltage of pure N2 is higher than that of SF 6 /N 2 gas mixtures, that is to say, the negative synergistic effect appeared. Under positive lightning impulse, the influence of gas pressure on 50% breakdown voltage is weaker, but the N-curve characteristic appeared because of the effect space charge. Meanwhile, the negative synergistic effect also appeared under positive lightning impulse at high gas pressure. The experimental results indicate that the breakdown time delay of pure N2 is much longer than that of SF 6 /N 2 gas mixtures with 1% SF 6 mixing ratio because of the obvious difference between the formative time delay of N2 and SF 6 /N 2 gas mixtures. The phenomenon of breakdown time delay demonstrates that the discharge form of N 2 is different from that of SF 6 /N 2 gas mixtures, that is to say, the streamer propagation changes to streamer-leader propagation.