Effect of Gas Content on Surface Charge Accumulation of Epoxy Insulator in C4F7N/CO2/O2 Mixture Under AC Voltage

Perfluoroisobutyronitrile (C4F7N) has emerged as a promising SF6 alternative due to its superior dielectric properties and acceptable environmental impact. However, the gas–solid interfacial charge accumulation behavior in such gas mixtures requires in-depth and systematic investigation. This study investigated the surface charge accumulation behavior on scaled disc insulators in C4F7N/CO2/O2 mixtures under AC voltage. By constructing a high-precision surface charge measurement platform, the influence mechanisms of varying gas composition ratios of C4F7N (2–14%) with fixed O2 content and O2 (2–14%) with fixed C4F7N content on charge accumulation were analyzed. The results demonstrated that increasing C4F7N content significantly suppresses surface charge accumulation. When the C4F7N concentration rises from 2% to 14%, the maximum positive/negative charge densities decrease by 46.58% and 22.22% in the absence of metal particles. The suppression effect is more pronounced with the metal particle present, where the reductions in positive/negative charge densities reach 61.90% and 23.71% under the same conditions. In contrast, variations in O2 content exhibit a weaker impact on charge accumulation, showing no consistent regulatory effect within the 2–14% range. By comparing charge distribution patterns under different gas compositions, it is revealed that C4F7N suppresses gas ionization primarily by enhancing electronegativity, while O2 exhibits negligible influence on charge transport. This study provides critical experimental evidence for optimizing gas ratios and insulation design in AC GIS equipment.