EFFECT OF WATER-VAPOR ON THE DISCHARGE REGIMES AND THE DEVIATIONS FROM SIMILARITY LAW IN COMPRESSED SF6 FOR POSITIVE POLARITY

The breakdown mechanisms in dry and moist compressed SF6 for inhomogeneous and quasi-homogeneous field configurations for positive polarity are dealt with. Deviations from similarity law are ascribed to the impurities present in SF6, mainly water vapour. It is experimentally shown that these impurities induce a change in the discharge modes. Adequate breakdown criteria are formulated depending on the degree of purity of the gas and/or the non-uniformity of the field distribution. In particular, in pure SF6, a direct streamer breakdown takes place when the mean electric field equals the critical field. The leader breakdown regime corresponds to the build-up of the critical avalanche. This study confirms the rejection of the effect of the micro-protrusions present on the electrode surfaces.