Study on the thermal decomposition characteristics of C4F7N–CO2 mixture as eco-friendly gas-insulating medium

The authors explored the thermal decomposition characteristics of perfluoroisobutyronitrile–carbon dioxide (C4F7N–CO2) gas mixture as eco-friendly dielectric medium. The main by-products and decomposition mechanism of C4F7N–CO2 gas mixture under different temperature and gas pressure conditions were revealed and analysed. It was found that the thermal decomposition of 6%C4F7N–94%CO2 gas mixture starts at about 350°C (0.15 MPa), producing C3F6 and CO first. Some other characteristic by-products such as CF4, C2F6, CF3CN, COF2 and (CN)2 could also be detected at higher temperature. The yield of C3F6, (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, C3F8, COF2 and CF3CN increased with temperature (350–550°C). The generation of CF4 and C2F6 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 C4F7N–CO2 gas mixture slowed down with the increase of gas pressure, indicating that C4F7N–CO2 gas mixture is quite suitable used at high-pressure equipment, especially high-voltage devices such as gas insulated switchgear.