Insulation characteristics of gas mixtures including perfluorocarbon gas

This paper describes discharge properties of N ₂ and CO ₂ -based gas mixtures including a perfluorocarbon (PFC) gas such as CF ₄ , C ₃ F ₈ and c-C ₄ F ₈ under non-uniform field. The mixture ratio between a base gas of N ₂ or CO ₂ and the additive PFC gas was fixed as 9:1; namely, 90%N ₂ /10%PFC or 90%CO ₂ /10%PFC gas mixture. The PFC gases have even smaller global warming potential (GWP) than SF ₆ gas and have good insulation properties as SF ₆ gas. Thus, PFC gas mixture is expected to be a SF ₆ 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 V _PD and breakdown voltage V _B properties between N ₂ and CO ₂ -based gas mixtures, as well as between the additive gas of PFC and SF ₆ 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 V _PD and V _B characteristics of N ₂ and CO ₂ -based gas mixtures differed considerably, especially the gas pressure dependence of V _B (so-called the N shape characteristics). V _B characteristics of N ₂ -based gas mixture including c-C ₄ F ₈ proved to be excellent within the test conditions over the wide gas pressure region, showing the maximum breakdown voltage. In terms of V _PD properties, CO ₂ -based gas mixture had an advantage over N ₂ -based gas mixture due to higher V _PD . Furthermore, we discussed the synergy effects of V _PD and V _B for N ₂ and CO ₂ -based gas mixtures using the index R _n which was defined to quantify the degree of the effect. R _n for CO ₂ -based gas mixture was higher than that of N ₂ -based gas mixture.