Effects of surface roughness on DC and AC surface flashover performances of epoxy composites in SF6/N2 mixtures

Surface flashover of the epoxy insulator is one of the urgent insulation issues in SF6/N2 filled gas-insulated equipment, causing irreversible insulation failure and harming the secure operation of power systems. However, the effects of surface roughness, which has always been changed in actual equipment, on flashover performances have not been comprehensively illustrated until now. In this work, the DC and AC surface flashover voltages of roughed epoxy specimens with a roughness range of 1–7 μm are investigated in 30%SF6–70%N2, and some supporting analysis measurement, including trap, conductivity, surface charging, and partial discharge has been carried out to clarify the how surface roughness affects flashover under steady or transient electric fields. The DC and AC flashover voltages of 7 μm roughness specimens are 28.34 and 16.85 kV, respectively, exhibiting a 28.01% increase compared to those of 1 μm roughness specimens under DC conditions, but they decrease by 12.60% under AC field. The roughed surface introduces shallow traps, inducing charge dissipation by increasing surface conductivity. Moreover, the extended discharge path can also elevate flashover voltage. Both surface charge transport and extended path effects cause DC flashover voltage to increase with roughness. Under AC conditions, partial discharge rather than surface charging dominates the flashover under the transient field, and partial discharge is accelerated by protrusions on the epoxy surface, leading to a decrease in AC flashover voltage with roughness. These findings enable the development of a roughness control strategy for practice insulators in gas-insulated equipment.