Insulation characteristics of gas mixtures including perfluorocarbon gas

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This paper describes discharge properties of N2 and CO2-based gas mixtures including a perfluorocarbon (PFC) gas such as CF4, C3F8 and c-C4F8 under non-uniform field. The mixture ratio between a base gas of N2 or CO2 and 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 SF6 gas and have good insulation properties as SF6 gas. Thus, PFC gas mixture is expected to be a SF6 substitute 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 VPD and breakdown voltage VB properties between N2 and CO2-based gas mixtures, as well as between the additive gas of PFC and SF6 gas, 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 VPD and VB characteristics of N2 and CO2-based gas mixtures differed considerably, especially the gas pressure dependence of VB (so-called the N shape characteristics). VB characteristics of N2-based gas mixture including c-C4F8 proved to be excellent within the test conditions over the wide gas pressure region, showing the maximum breakdown voltage. In terms of VPD properties, CO2-based gas mixture had an advantage over N2-based gas mixture due to higher VPD. Furthermore, we discussed the synergy effects of VPD and VB for N2 and CO2-based gas mixtures using the index Rn which was defined to quantify the degree of the effect. Rn for CO2-based gas mixture was higher than that of N2-based gas mixture.