Effect of trace SF6 on negative corona characteristics in SF6/N2 gas mixtures under DC voltages

Corona discharge will affect the performance of insulating gas, threatening the safe and reliable operation of gas-insulated power equipment. In this paper, we investigate the negative corona characteristics in SF6/N2 gas mixtures with trace SF6 [φ(SF6) ≤ 1%] under DC voltages. With decreasing SF6 content, two types of the discharge–stage transition process as a function of mean current are observed. One process is from the Townsend stage, through the pulse stage, to the breakdown stage; the other one presents a normal-glow stage between the pulse stage and the breakdown stage. The threshold of SF6 content which can change the discharge–stage transition process in SF6/N2 gas mixtures is between 0.1% and 0.15%. The lower limit of positive-ion flux in the normal-glow stage is 6–7 orders of magnitude higher than the upper limit of positive-ion flux in the Townsend stage. The variation in pulse characteristic parameters, including pulse–time parameters and pulse amplitude, with increasing SF6 content is investigated. The pulse–time parameters show a trend of decreasing first and then saturation with increasing SF6 content, while the pulse amplitude shows a slightly increasing trend. Moreover, the “return stroke” phenomenon of negative corona in SF6/N2 gas mixtures is observed through discharge light emission images. The “return stroke” phenomenon does not necessarily occur in the normal-glow stage, resulting from competition between the position of the negative-ion cloud and the negative-ion flux.