Influence of electric field non-uniformity on discharge characteristics in SF<inf>6</inf>/N<inf>2</inf> gas mixtures under power frequency voltage

Abstract

In recent years, SF 6 /N 2 gas mixtures with low SF 6 mixing ratio was getting used as insulating medium in gas-insulated metal-enclosed transmission line in order to replace pure SF 6 . For the application of SF 6 /N 2 gas mixtures in power equipment, this paper studied on the breakdown characteristics of SF 6 /N 2 gas mixtures with low SF 6 mixing ratio in different electric field non-uniformity under power frequency voltage, and the characteristic of partial discharge initial voltage was studied in non-uniform electric field. In order to study the breakdown characteristic of SF 6 /N 2 gas mixtures under high voltage grade, a fully enclosed power frequency voltage test device was set up. The sphere-plane and rod-plane electrodes were used to simulate the slightly non-uniform field and local-field enhancement in GIS or GIL. The research indicates that the breakdown voltage of SF 6 /N 2 gas mixtures increases linearly with increase of gas pressure in slightly non-uniform electric field. With the increase of the electric field non-uniformity, the linearity of breakdown voltage weakens and the stable corona discharge appears. Because of the space charge effect, the N-curve characteristic of breakdown voltage appears in SF 6 and its mixtures. When electric field non-uniformity exceeds a certain value, the stable corona discharge appears before the electric breakdown of electrodes gap. But the breakdown occurs immediately with the appearance of corona when the gas pressure exceeds a certain value. Meanwhile, the synergistic effect was strengthened and then weakened. And the amplitude and range of N-curve extend when electric field non-uniformity increased.