Influence of voltage polarity on breakdown of SF6 rod-plate gap under VFTO and standard LI

Gas insulation switchgear (GIS) has been widely used in electrical power systems for decades due to its good compactness and tightness, slight impact on the environment, and long maintenance cycle. However, it has been noticed that the failures of GIS insulation due to very fast transient overvoltages (VFTOs) increases with the increase of voltage levels. In this paper, a generating system of VFTO and standard lightning impulse (SLI) was established. Using the system, the breakdown characteristics of SF₆ gas gaps under VFTO and SLI were studied. The results show that SF₆ gap has polarity effect in both quasi-uniform field and highly inhomogenous electric field. Under quasi-uniform field, the 50% breakdown voltages under VFTO and SLI with positive polarity are higher than those with negative polarity. But for the highly inhomogenous electric field, the polarity effect is remarkable and polarity reversion phenomenon is observed. When the pressure of SF₆ is lower, the 50% breakdown voltages under SLI with negative polarity are higher than those with positive polarity. Nevertheless, the 50% breakdown voltages under SLI with positive polarity are higher than those with negative polarity when the pressure of SF₆ exceeds a critical pressure. The critical pressure decreases with the increase of the electric field non-uniform factor. Based on the analysis, it is concluded that the reasons for polarity effect can be explained by the migration and proliferation of space charge.