Investigation of Ionization Characteristics of C4F7N-N2 Gas Mixtures as Alternative Gas to SF6 by Steady-state Townsend Method

In order to evaluate the ionization characteristics of C4F7N-N2 gas mixtures, an experimental apparatus based on steady-state Townsend (SST) method was set up. The normalized ionization coefficient, attachment coefficient as well as effective ionization coefficient of pure SF6 and N2 gases were measured on this SST apparatus and compared with the experimental results reported in literatures, which verified the reliability of the apparatus. Thereby, the ionization characteristics of 9%C4F7N-91%N2 and 20%C4F7N-80%N2 gas mixtures were studied, respectively, by measuring currents (~pA) of electronic avalanche in the non-self sustained discharge stage using the SST apparatus under the conditions of 20℃ and the reduced electric field varying from 200 Td to 500 Td. It is found that the critical insulation strength of 20%C4F7N-80%N2 is 407.43 Td, however, the critical insulation strength of 9%C4F7N-91%N2 is 305.93 Td. These results show that when there is no self-sustaining discharge in the dielectric, the critical insulation strength of 9%C4F7N-91%N2 gas mixture is about 85% of that of pure SF6 gas. However, the critical insulation strength of 20%C4F7N-80%N2 gas mixture is about 1.15 times that of pure SF6 gas, which shows a better insulation performance than SF6 gas. © 2019, High Voltage Engineering Editorial Department of CEPRI. All right reserved.