The lifetime prediction of epoxy composites used in solid state transformer under high-frequency bipolar square wave fields

Solid-state transformers (SSTs) serve as critical enabling components in smart grids and future artificial intelligence infrastructure. The epoxy composites employed in SSTs are required to endure the synergistic stresses of high-frequency bipolar square-wave voltages and elevated temperatures. Nevertheless, the absence of insulating lifetime assessment methodologies under such extreme operating conditions severely impedes the development of epoxy composites with high thermal conductivity and high breakdown strength. This work systematically investigates the aging behavior of epoxy composites subjected to combined high-frequency and high-temperature stresses. By conducting comprehensive performance characterizations on specimens aged for 200 h, 400 h, 600 h, 800 h, and 1000 h, the evolution of microstructure, trap characteristics, and breakdown strength of aged epoxy composite were obtained. Furthermore, the intrinsic correlations between these parameters and the dominant aging factors are elucidated. Based on these findings, an empirical lifetime prediction model for epoxy composites in SST is established.