The Reversal of SF6/N2 Gas Mixtures Polarity Effect Under Lightning Impulse

In recent years, SF6/N-2 gas mixture gains much popularity as an alternative insulating medium of pure SF6 gas among gas-insulated equipment because of the greenhouse effect and poor economy effectiveness of the latter. This paper investigated the breakdown characteristics of SF6/N-2 gas mixture with low SF6 mixing ratio in slightly non-uniform electric field which is similar to the gas-insulated metal-enclosed transmission line ( GIL) under positive and negative lightning impulse, and the reversal of the polarity effect was found. The research indicates that the breakdown voltage under negative lightning impulse is higher than that under positive lightning impulse in low gas pressure, but when pressure exceeds a critical value, the result converses. The rise of SF6 mixing ratio will lead to the decrease of the critical value of gas pressure. The result is significant for guiding the application of the electric power equipment with SF6/N-2 gas mixtures such as GIL, and it also plays a vital role in the future study of gas discharge mechanism of SF6/N-2 gas mixtures.