Deterioration Characterization of Epoxy Resin in Wall Bushings of Operational Air-Insulated Switchgears

The epoxy resin insulation of wall bushing plays a critical role in high-voltage air-insulated switchgear (AIS). Breakdown occurs frequently at short edge of flange, resulting in serious erosion of AIS. Although accelerated aging and deterioration of epoxy resin have been well investigated in laboratory, the generation and developing of defects, as well as its electrical performance evolution have not been systematically realized in long-term operational wall bushings, forming obstacles for their accurate deterioration assessment and maintenance. Here in this work, the electrical field distribution under operating condition and typical fault condition of internal shielding virtual connection is investigated, and electric field distortion on short edge of flange is analyzed. The extreme electric distortion leads to defect development from blowholes to cracks, characterized by ultrasonic detection on wall bushings with effective insulation, with external burns, and with serious insulation damages, extracted from operating conditions. Combined analysis of ultrasonic and partial discharge results indicates that the deterioration of insulation performance correlates with an increase in defect type and density, as well as discharge capacity. Furthermore, the dielectric properties and breakdown strength are tested, demonstrating significant increase in permittivity and dielectric loss, and largely reduced breakdown strength with deterioration. This study reveals the dielectric and electrical performance evolution of epoxy resin under long-term deterioration, and provides reference for accurately evaluating the insulation status of wall bushing in engineering operations.