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The authors explored the thermal decomposition characteristics of perfluoroisobutyronitrile-carbon dioxide (C 4 F 7 N-CO 2) gas mixture as eco-friendly dielectric medium. The main by-products and decomposition mechanism of C 4 F 7 N-CO 2 gas mixture under different temperature and gas pressure conditions were revealed and analysed. It was found that the thermal decomposition of 6%C 4 F 7 N-94%CO 2 gas mixture starts at about 350°C (0.15 MPa), producing C 3 F 6 and CO first. Some other characteristic by-products such as CF 4 , C 2 F 6 , CF 3 CN, COF 2 and (CN) 2 could also be detected at higher temperature. The yield of C 3 F 6 , (CN) 2 increased with the temperature (lower than 450°C) first and then decreased when it reached to 500°C. While the yield of CO, C 3 F 8 , COF 2 and CF 3 CN increased with temperature (350-550°C). The generation of CF 4 and C 2 F 6 begins at temperatures higher than 500°C, which can be used as the feature component of severe overheating fault. The thermal decomposition amount and by-products yield of C 4 F 7 N-CO 2 gas mixture slowed down with the increase of gas pressure, indicating that C 4 F 7 N-CO 2 gas mixture is quite suitable used at high-pressure equipment, especially high-voltage devices such as gas insulated switchgear.

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