Surface Discharge Mechanism of Epoxy/Al2O3 Composites in SF6 Under Long-Term DC and AC Stresses
To investigate the impact of long-term voltage stress on the surface charge accumulation and discharge characteristics of insulating materials, Epoxy/Al2O3 composites were subjected to DC and AC voltage application tests for durations of seven and 30 days. By integrating surface charge measurements, isothermal surface potential decay (ISPD) analysis, and scanning electron microscopy (SEM), the regulatory mechanisms of surface charge accumulation and trap distribution parameters on discharge behavior were analyzed. The results indicate that under long-term DC voltage, continuous unipolar charge injection induces microdefects such as grooves and pores on the material surface, leading to a significant increase in deep trap density. This enhancement in surface charge binding capability results in an increase in charge density with prolonged voltage application, which subsequently intensifies electric field distortion and partial discharge (PD) activity. On the contrary, the surface charge density under AC voltage is generally lower and exhibits an increasing trend from the high-voltage electrode toward the grounded electrode. Notably, the sample stressed for seven days exhibited the lowest surface charge density yet the most intense PD activity. This phenomenon is attributed to an increase in shallow trap density, which enhances surface conductivity and carrier mobility. Conversely, after 30 days of stress, the formation of deep traps restores the charge binding capability. Furthermore, the significant accumulation of negative charges near the grounded electrode under AC conditions is identified as a critical factor potentially triggering surface discharge.