Characteristics of VFTO generated in a GIS with SF6-N 2 gas mixture as the insulating medium

This paper deals with the experimental characteristics of VFTO in pure N₂, pure SF₆ and SF₆-N₂ mixtures. The investigations are performed using a laboratory model bus duct. A capacitive type voltage sensor was used to measure the VFTO peak magnitudes and temporal characteristics. Measurements were carried out in N₂, SF₆ and SF₆-N₂ mixtures wherein the SF₆ concentration was varied from 10% to 40% over a pressure range of 1 bar to 5 bars. The results from the VFTO characteristics obtained for SF₆ and SF₆-N₂ mixtures show similar trends in variations. The levels of surge peak magnitudes are less than 2 p,u. for all the cases considered in the experimental pressure range. In N₂, the peak magnitude remains almost constant with increasing pressure but for pure SF₆ , it increases almost linearly with pressure. On the other hand, for the gas mixtures, the difference in peak magnitudes is not significant at pressures between 2 to 4 bars. The occurrence of corona stabilization during breakdown of the gap is thought to be the cause for this behavior