Characteristics of VFTO generated in a GIS with SF/sub 6/-N/sub 2/ gas mixture as the insulating medium

This paper deals with the experimental characteristics of VFTO in pure N/sub 2/, pure SF/sub 6/ and SF/sub 6/-N/sub 2/ 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 N/sub 2/, SF/sub 6/ and SF/sub 6/-N/sub 2/ mixtures wherein the SF/sub 6/ 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 SF/sub 6/ and SF/sub 6/-N/sub 2/ 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 N/sub 2/, the peak magnitude remains almost constant with increasing pressure but for pure SF/sub 6/, 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.