Gas insulated switchgear (GIS) inevitably experiences switching overvoltages during operation. To investigate the influences of the combined action of alternating current (AC) and switching impulse (SI) voltages on the insulation performance of epoxy insulators

this paper studies the effects of AC voltage amplitude on the surface discharge excitation characteristics in SF6/N2 gas under AC/SI superposition voltage. The focus is placed on key discharge parameters

including the development trend of creeping discharge

discharge quantity

inception time of the first partial discharge (PD)

and discharge duration. The results reveal that

firstly

with increasing AC voltage amplitude

the probability of creeping discharge during the SI stage significantly increases

accompanied by a slight increase in discharge numbers and an earlier appearance of the initial PD. During the AC stage

both the probability and quantity of discharge events increase markedly

and the discharge duration is extended accordingly. Secondly

the occurrence of creeping discharge under SI voltage enhances the severity of subsequent PDs during the AC. Lastly

the phase angle of the superimposed voltage has a notable effect on the PD behavior. For positive SI polarity

PD activity at a 135° phase is more intense than at 45°

while for negative SI polarity

the PD at 315° is more pronounced than at 225°. This study provides valuable insight into the surface discharge characteristics under AC/SI superimposed voltage conditions and offers theoretical supports for the standardization of combined voltage tests involving AC and impulse voltages.