Effect of O2 content on AC surface flashover and surface degradation of Al2O3-filled epoxy resin in C4F7N/CO2/O2 ternary gas mixtures

Introducing a controlled amount of O 2 into the environmentally friendly C 4 F 7 N/CO 2 gas can effectively suppress the deep decomposition of C 4 F 7 N, enhance the initial flashover voltage of the mixture. Consequently, this ternary gas mixture, which balances insulation performance with environmental compatibility, has already found application in certain SF 6 replacement schemes. Nevertheless, the effect of O 2 content on flashover behavior at the gas–solid interface has not been thoroughly investigated, and the O 2 proportion in most existing ternary gas formulations remains poorly defined. To determine an optimal oxygen mixing ratio, a gas–solid interface discharge experimental setup was designed and implemented, enabling a systematic study of the flashover characteristics of C 4 F 7 N/CO 2 /O 2 ternary mixtures under varying gas pressures and O 2 concentrations, as well as their influence on the surface degradation of epoxy resin. By adjusting the O 2 volume fraction from 0 to 13% and the gas pressure from 0.1 to 0.4 MPa, the modulation effect of the mixed gas on the initial flashover voltage and its correlation with the evolution of the insulating surface morphology were analyzed. The results indicate that the appropriate addition of O 2 improves the insulation performance of the mixture, with optimal flashover behavior observed at 8%O 2 under relatively high gas pressures. However, when the O 2 content exceeds 10%, the accumulation of degradation traces is intensified, leading to pronounced carbonization and thermal damage. Specifically, continuous high-voltage flashover severely ablates the epoxy matrix, exposing internal alumina particles; under these conditions, the introduced oxygen significantly amplifies the discharge’s oxidative and thermal effects, thereby exacerbating interfacial carbonization and overall insulation degradation.