Comparison of DC PDIV and BDV of GIS defects between SF6 gas and C4F7N/CO2 gas mixture

Abstract

SF6 gas has been used as an insulation material for gas insulated switchgear (GIS) due to its excellent dielectric strength, but C4F7N/CO2 gas mixture is emphasized as to replace SF6 because of global warming. In AC GIS, the dielectric strength of the gas mixture tends to be slightly lower than the SF6 gas, but it is reported that the gas mixture can be applied to the present gas insulating system. However, studies on the application of gas mixtures to DC GIS are insufficient. Therefore, in order to ensure the reliability and safety of the gas mixture in DC GIS, it is necessary to study the insulation properties of the eco-friendly gas mixture. In this paper, two insulating gases (SF6 and C4F7N/CO2 mixture) were investigated. DC partial discharge inception voltage (PDIV) and breakdown voltage (BDV) tests of typical defects of GIS (Protrusion Electrode, Floating Electrode, Metallic Particles) were performed by selecting the gas pressure as up to 0.3 Mpa. As a result, the BDV of the C4F7N/CO2 mixture was 65-80% from the SF6, and it was confirmed that the C4F7N/CO2 mixture caused partial discharge and subsequent insulation breakdown under low pressure conditions. The dielectric strength of the gas mixture was lower, but it showed comparable to SF6 gas at high pressure.