Study of fast transients generated in a GIS bus duct in compressed SF6-N2 gas mixtures containing lower percentages of SF6

This paper deals with the study of very fast transient overvoltages (VFTO) with SF₆-N₂ gas mixture as the insulating medium, but with lower percentages of SF₆ in the mixture. The investigations are performed using a laboratory model bus duct. A capacitive voltage sensor was used to measure the peak VFTO and temporal characteristics. Measurements were carried out in SF₆N₂ gas mixtures wherein the concentration of SF ₆ was varied from 0 to 20% over a pressure range of 1 to 5 bars. The results are then compared with those obtained for pure SF₆. The VFTO characteristics obtained for SF₆ and SF₆-N₂ mixtures show similar trends in variations. The levels of surge peak magnitudes are less than 2.0 p.u. In the experimental pressure range in all the cases. In N₂, the VFTO level remains almost constant with increase in pressure whereas in the case of mixtures, the VFTO level was found to increase with increase in pressure as well as with increase in the percentage of SF₆ in the mixture. In mixtures, no significant difference in the VFTO levels is seen between 2-4 bars, although this fact is more pronounced at percentages of SF₆ higher than 5%. This can be attributed to the occurrence of corona stabilization in the breakdown process