Surface aging mechanism and insulation degradation causes of epoxy/anhydride/SiO2 composite materials for current transformers in switchgear

The epoxy-casting current transformer (EPCT) uses epoxy/anhydride/SiO2 composite as the external insulation material and the surface of the composite is prone to irreversible aging under harsh conditions inside the switchgear, leading to a significant reduction in surface insulation properties, thereby endangering the safety of the power system. In this paper, both unaged and naturally aged EPCT samples were prepared. Through insulation property testing, microscopic morphology observation, and substance composition testing, the aging mechanism and causes of insulation degradation were analyzed. The results show that under the influence of moisture, discharge, heat, and oxygen, the material underwent severe degradation through swelling of the crosslinked system, as well as the oxidation, hydrolysis, dehydration and carbonization of the organic matter. Inorganic salts of the surface contamination promoted the aging of the material. Natural aging led to a significant decline in surface resistivity, flashover voltage and partial discharge inception voltage of the material, and the originally smooth and dense surface was severely damaged with the structure becoming rough and loose, leading to the formation of cracks, pores, and carbonization products. The upper surface suffered more severe degradation than the lower surface. The surface resistivity of EPCT showed an exponential negative correlation with the total relative content of inorganic salts and carbonization products. Moisture dissolved the inorganic salts in the contamination and created conductive pathways between the isolated dispersed contamination particles, leading to a synergistic effect between inorganic salts and carbonization products, which caused a significant decrease in the surface resistivity of the aged EPCT.