Insulation characteristics of gas mixtures including perfluorocarbon gas

This paper describes discharge properties of N2and CO2-based gas mixtures including a perfluorocarbon (PFC) gas such as CF4, C3F8and c-C4F8under non-uniform field. The mixture ratio between a base gas of N2or CO2and the additive PFC gas was fixed as 9:1; namely, 90%N2/10%PFC or 90%CO2/10%PFC gas mixture. The PFC gases have even smaller global warming potential (GWP) than SF6gas and have good insulation properties as SF6gas. Thus, PFC gas mixture is expected to be a SF6substitute without highly pressurizing the gas over the conventional pressure of 0.5 to 0.6 MPa. In this study, in order to compare the partial discharge (PD) inception voltage VPDand breakdown voltage VBproperties between N2and CO2-based gas mixtures, as well as between the additive gas of PFC and SF6gas, we investigated these properties of the gas mixtures with a needle to plane electrode under ac high voltage application. The gas pressure was changed from 0.1 to 0.6 MPa. As a result, it was found that VPDand VBcharacteristics of N2and CO2-based gas mixtures differed considerably, especially the gas pressure dependence of VB(so-called the N shape characteristics). VBcharacteristics of N2-based gas mixture including c-C4F8proved to be excellent within the test conditions over the wide gas pressure region, showing the maximum breakdown voltage. In terms of VPDproperties, CO2-based gas mixture had an advantage over N2-based gas mixture due to higher VPD. Furthermore, we discussed the synergy effects of VPDand VBfor N2and CO2-based gas mixtures using the index Rnwhich was defined to quantify the degree of the effect. Rnfor CO2-based gas mixture was higher than that of N2-based gas mixture.