Experimental Study and 3-D PIC-MCC Simulation of DC Discharge in Various Concentrations of SF6 Admixture

The impact of the gas mixing ratio on the insulation strength refers to the synergistic behavior of mixed gas. In this work, synergistic effects of various SF6 admix (SF6/N2, SF6/dry air, and SF6/Ar) are tested using HV co-axial sparkgap switch with different % by vol of SF6 under nonuniform field background (point-plane configuration). The experimental work is performed with 5%, 10%, 25%, and 50% by vol of SF6 for pressure levels 34.5–206.7 kPa in a gap length of 4 mm. For the subjected experimental constraint, SF6 admix with Ar, N2, and dry air exhibits a positive synergism. The breakdown voltage (BDV) of the SF6/N2 and SF6/dry air mixture with 5% SF6 is found to be ~60% of pure SF6. Furthermore, SF6/N2 admix shows a saturated growth trend with an increase in % by vol. of SF6. The binary mixtures of SF6 with N2, dry air, and Ar exhibit an increasing trend in the breakdown characteristics with an increase in the concentration levels of SF6. Besides the experimental study, a discharge model has been developed using 3-D particle-in-cell/Monte Carlo collision (PIC/MCC) code for binary mixtures of SF6 with N2 and Ar for the same volumetric proportions at which the experimentation work was carried out. In the simulations, ionization by interactions of SF6 with N2 and Ar atoms are considered, whereas dissociative ionization and plasma chemistry has been ignored. The BDV, electron number density, and temporal variation of the electric field during the discharge process are obtained through numerical simulations. The calculated BDV is found to have a good agreement with experimental breakdown data. From this study, one can predict the BDV of SF6 admix, thereby providing a reference for designing and manufacturing power equipment using SF6 admix.