Mechanisms for DC Breakdown Strength Degradation in Epoxy/Glass Fiber Composites Under Long-Term Aging

In modern power systems, the proportion of epoxy/glass fiber (EP/GF) composites that served as the primary insulation composites in electrical equipment is steadily increasing. These devices suffer long-term electrical, thermal, and vibrational stresses during operation, leading to degradation and finally insulation failures. However, the underlying electrical-thermal–vibrational aging mechanisms of the dc breakdown performance of EP/GF composites remain unclear. In this work, combined electrical-thermal–vibrational aging experiments (1080 h) were performed on the prepared EP/GF composites. Infrared spectrum, dielectric spectrum, surface potential decay (SPD), and breakdown tests were used to characterize the functional groups, side-chain dynamics, trap levels, and dc breakdown strength of aged EP/GF composites. The research indicates that after long-term aging, the dc breakdown strength of EP/GF composites first increases by 5.8% at 72 h and then decreases by 20.6% at 1080 h. The aging of EP/GF composites is divided into recrosslinking and degradation stages. The recrosslinking stage enhances the dc breakdown strength of the EP/GF composites by reducing free volume before 72 h. While in the degradation stage, chain breaking and oxidation of epoxy chains steadily occur, introducing abundant short chains and polar groups (carbonyl and ester groups), improving free volume fraction, and accelerating chains’ dynamic. The improved free volume fraction accelerates electron avalanche in the electric field, stimulating charges to escape from traps and causing dc breakdown strength to considerably decrease during 72–1080 h.