Partial Discharge Behaviors Excited by High-Frequency Square Pulse Voltage in Cast Resin Insulation With Thermal-Induced Defect

The compact design of the cast resin high-frequency transformers (HFTs) allows the insulation of fine structures withstand severe high-frequency square pulse voltage stress and thermal stress. The partial discharge (PD) behaviors in the presence of thermal-induced defects are investigated. The phase-resolved PD (PRPD) pattern showed that the majority of PDs occurs on the rising and falling edges of the square pulse voltage. The PD inception voltage (PDIV), PD magnitude, and PD repetition rate increase with frequency at very low frequency and decrease with frequency at high frequency. Long charge injection time at very low frequency and the heat effect at high frequency cause the diminution of these PD parameters. The PD and breakdown characteristics are examined under progressive temperature conditions, indicating an early PD degradation and electrical failure phenomenon below the glass transition temperature (T-g). Microscopic observation revealed the formation of delamination gap defects due to the inconsistency of thermal expansion between the cast insulation and metal, which considered the cause of PDs. The decohesion model is employed to describe the structural evolution and the formation of delamination gap with varying temperatures. The model demonstrates an early damage mechanism for interfacial cohesion condition at medium temperatures and temperatures close to T-g . Additional experiment enables the inference that the release of local interfacial stress under progressive temperature may lead to the formation of damaged interfaces or microcracks, which can trigger occasional PD or instantaneous filamentary breakdown.