| Summary: | As one of the alternative insulation gases, C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub> 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<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub> 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<sub>4</sub>F<sub>7</sub>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<sub>4</sub>F<sub>7</sub>N/91.00%CO<sub>2</sub> respectively. The results indicated that the (E/N)lim have not significant change on the SST experimental conditions. Meanwhile, a comparison with SF<sub>6</sub> gas indicates that 20.02% C<sub>4</sub>F<sub>7</sub>N/79.98% CO<sub>2</sub> has a similar insulation performance with (E/N)lim as 358.82 Td. However, when the liquefaction temperature is considered, the C<sub>4</sub>F<sub>7</sub>N/CO<sub>2</sub> gas mixtures with 9.00% C<sub>4</sub>F<sub>7</sub>N could be considered as an appropriate substitute gas for SF<sub>6</sub>, because the critical electric field of this gas mixture is over 70% of SF<sub>6</sub> gas and could be used under -15°C at 0.7 MPa.
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