The normalized Townsend first ionization coefficient α/N and normalized attachment coefficient η/N in pure C4F7N were measured by using the steady-state Townsend (SST) method for a range of reduced electric fields E/N from 750 to 1150 Td at room temperature (20 °C). Meanwhile, the effective ionization coefficients are obtained. All SST experimental results show good agreement with pulsed Townsend (PT) experiment results. Comparisons of the critical electric fields of C4F7N with SF6 and other alternative gases such as c-C4F8 and CF3I indicate that C4F7N has a better insulation performance with a much higher normalized critical electric field at 959.19 Td.

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The electron swarms parameters of trifluoromethanesulfonyl fluoride (CF 3 SO 2 F) with a low value of global warming potential (GWP) of 3678 were measured for a range of reduced electric fields (per gas density N) E/N from 400 to 750 Td at room temperature (20 °C). The discharge parameters such as the reduced Townsend first ionization coefficient α/N, the reduced attachment coefficient η/N, and the reduced effective ionization coefficients (α−η)/N are obtained by using the steady-state Townsend method. The comparison of the experimental results with those of SF 6 , a kind of the most widely used insulation gas, indicates that CF 3 SO 2 F has a better insulation performance than SF 6 because of a much higher reduced critical electric field of 578.8 Td.

As one of the alternative insulation gases, C 4 F 7 N/CO 2 gas mixtures have attracted much attention recently. In this study, the normalized Townsend first ionization coefficient α/N and the normalized attachment coefficient η/N were measured in C 4 F 7 N/CO 2 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 C 4 F 7 N studied were 4.99, 7.00, 9.00, 13.13 and 20.02%, and the gas pressure were 500 Pa at 20 • 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%C 4 F 7 N/91.00%CO 2 respectively. The results indicated that the (E/N) lim have not significant change on the SST experimental conditions. Meanwhile, a comparison with SF 6 gas indicates that 20.02% C 4 F 7 N/79.98% CO 2 has a similar insulation performance with (E/N) lim as 358.82 Td. However, when the liquefaction temperature is considered, the C 4 F 7 N/CO 2 gas mixtures with 9.00% C 4 F 7 N could be considered as an appropriate substitute gas for SF 6 , because the critical electric field of this gas mixture is over 70% of SF 6 gas and could be used under-15 • C at 0.7 MPa. INDEX TERMS C 4 F 7 N/CO 2 , ionization coefficient, attachment coefficient, critical electric field, steady-state Townsend method.

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