Development of SF6-Free 72.5 kV GIS

This paper describes the fundamental insulation characteristics of environmentally friendly gases; dry air, N₂ and N₂ O₂ mixed gas. Conventional gas insulated switchgear (GIS) generally uses SF₆ as against over 72.5 kV class. We have proposed GIS with a vacuum circuit breaker against from 72.5 kV class to 170 kV class as SF₆-free GIS. Because the insulation gas of this type of SF6 -free GIS has the hardly unnecessary current interception performance, the optimal insulation gas can be selected based on fundamental insulation performance. From the standpoints of insulation performance and economical efficiency, we selected compressed dry air as the optimal insulation gas for this type of SF₆-free GIS. However, the dielectric strength of dry air is approximately one-third that of SF₆ gas. Therefore, to achieve roughly the same size as conventional GIS, some strategies must be employed to enhance insulation performance. This paper investigates the influence of gas pressure and a gas/solid hybrid insulation structure. The insulation performance of the gas/solid hybrid insulation structure using an insulation coating was found to be better than that of a barrier. Moreover, the dielectric strength of particles adhering to a spacer under the compressed dry air was about 1.5 times higher than that under N₂. Thus, the high insulation reliability of the gas/solid hybrid structure was high against particles was confirmed. In view of these investigative findings, this type of SF₆-free 72.5 kV GIS which was designed and manufactured, was confirmed to comply with the IEC standard.