Investigating the Influence of O2 on Surface Flashover Characteristics and Gas-Solid Interface Compatibility in C4F7N/CO2 Mixtures Under Negative DC Voltage

Previous studies have indicated that the addition of O2 can enhance the insulation performance and chemical stability of C4F7N/CO2 mixed gases during partial discharge to a certain extent. However, the influence of O2 on the surface flashover characteristics at gas-solid interfaces, as well as the compatibility and interaction mechanisms between gaseous and solid materials, remain poorly understood. Furthermore, there is a lack of empirical basis for determining the optimal proportion of O2 in most current ternary gas mixtures. To identify an appropriate O2 mixing ratio, we investigated the effects of different O2 concentrations on the flashover characteristics at the gas-solid interface of C4F7N/CO2 mixtures and analyzed the microscopic properties of the solid surfaces after flashover. Our findings show that O2 content has a minimal overall impact on the flashover voltage and current magnitude, suggesting that its influence on decomposition characteristics should be the primary consideration when developing mixture formulations. Notably, adding 2% to 4% O2 significantly suppresses the deposition on the solid surface, whereas increasing the O2 content beyond 6% leads to the formation of evident ablation pits or black solid deposits near the triple junction of gas, electrode, and insulation material, thereby compromising solid insulation. In summary, the addition of O2 intensifies the reaction between the epoxy resin and the C4F7N/CO2 mixture (also its decomposition products), thereby reducing the compatibility of gas-solid interface materials in the C4F7N/CO2 system.