Effect of Gas Content on Surface Charge Accumulation of Epoxy Insulator in C 4 F 7 N/CO 2 /O 2 Mixture Under AC Voltage

Perfluoroisobutyronitrile (C 4 F 7 N) has emerged as a promising SF 6 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 C 4 F 7 N/CO 2 /O 2 mixtures under AC voltage. By constructing a high-precision surface charge measurement platform, the influence mechanisms of varying gas composition ratios of C 4 F 7 N (2–14%) with fixed O 2 content and O 2 (2–14%) with fixed C 4 F 7 N content on charge accumulation were analyzed. The results demonstrated that increasing C 4 F 7 N content significantly suppresses surface charge accumulation. When the C 4 F 7 N 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 O 2 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 C 4 F 7 N suppresses gas ionization primarily by enhancing electronegativity, while O 2 exhibits negligible influence on charge transport. This study provides critical experimental evidence for optimizing gas ratios and insulation design in AC GIS equipment.