Very fast transient overvoltages in GIS with compressed SF6-N-2 gas mixtures

This paper discusses the characteristics of very fast transient overvoltage s (VFTO) in SF6-N-2 gas mixtures at different percentages Of SF6. A compari son of the VFTO characteristics of pure SF6 with those of pure N-2 is also presented, The investigations are performed using a laboratory model GIS bu s duct having a test gap used for simulating a switching event leading to t he generation of VFTO. A capacitive voltage sensor is used to measure the V FTO peak magnitude and temporal characteristics. Measurements were carried out at two different gap spacings (0.20 and 0.61 mm) over a pressure range of 100 to 500 kPa. VFTO characteristics for N-2, SF6 and SF6-N2 mixtures ob tained from the experiments show similar trends. The level of surge peak ma gnitude is <2.0 pu for all cases when the gap was 0.20 mm, but it reaches a maximum of 2.41 pu at 0.61 mm gap. At 0.20 mm gap, in SF6-N-2 mixtures, th e difference in peak magnitudes is not significant for 10% and 20% SF6 mixt ures (between 200 and 400 kPa) and also for pure SF6 and 40% SF6 (between 2 00 and 300 kPa), The occurrence of corona stabilization during breakdown of the gap may be the cause for such a behavior. Unlike the above observation s at 0.20 mm gap, at 0.61 mm gap, the peak magnitudes strictly increase wit h pressure for the pure gases and gas mixtures. At 0.20 mm. gap, the time t o breakdown of the gap is found to be almost constant in all cases. But at 0.61 mm gap, the time to breakdown is seen to be dependent on the mixture, pressure, and breakdown voltage, and this observation is in accordance with Toepler's spark law.