Optimum SF6-N2, SF6-air, SF6 -CO2 mixtures based on particle contamination

The design stress of sulphur hexafluoride Gas Insulated Switchgear (GIS) systems are limited by the harmful effects resulting from the almost inevitable presence of particulate contamination in GIS equipment. Mixing SF₆ with other gas is a possible way of improving GIS performance under particle contamination condition. This paper studies the breakdown of SF₆-gas mixture (SF₆-air, SF₆-N₂ and SF₆-C ₄) in the presence of contaminating particles under applied DC voltage. The effect of particle dimensions on the breakdown voltage and so on the particle contamination factor (PCF) is presented in this work. The breakdown voltage-pressure characteristics for SF₆-gas mixture with different fractional concentration of SF ₆ in mixture are carried out. Also, the particle contamination sensitivity is sought for different parameters and to obtain the optimum SF₆-gas mixtures for the case study. From this study it is concluded that some SF₆-gas mixtures have breakdown gradients comparable to that of pure SF₆ and are less sensitive to the presence of strong localized fields