Measurement of Ionization and Attachment Coefficients in C4F7N/CO2 Gas Mixture as Substitute Gas to SF6

As one of the alternative insulation gases, C4F7N/CO2 gas mixtures have attracted much attention recently. In this study, the normalized Townsend first ionization coefficient $\alpha /N$ and the normalized attachment coefficient $\eta /N$ were measured in C4F7N/CO2 gas mixtures by using the Steady-State Townsend (SST) method over a range of electric fields $E/N$ from 100 to 550 Td. The concentrations of C4F7N studied were 4.99, 7.00, 9.00, 13.13 and 20.02%, and the gas pressure were 500 Pa at $20^{\circ }\text{C}$ . The effective ionization coefficients and the critical electric field were obtained at 500, 1000 and 2000 Pa to investigate the effect of gas pressure on dielectric strength in 9.00%C4F7N/91.00%CO2 respectively. The results indicated that the $(E/N)_{\mathrm {lim}}$ have not significant change on the SST experimental conditions. Meanwhile, a comparison with SF6 gas indicates that 20.02% C4F7N/79.98% CO2 has a similar insulation performance with $(E/N)_{\mathrm {lim}}$ as 358.82 Td. However, when the liquefaction temperature is considered, the C4F7N/CO2 gas mixtures with 9.00% C4F7N could be considered as an appropriate substitute gas for SF6, because the critical electric field of this gas mixture is over 70% of SF6 gas and could be used under −15°C at 0.7 MPa.