Partial Discharge Aging of Epoxy Insulation at Different High-Frequency Pulse Parameters and Environmental Stresses

The voltage stress on high-frequency transformers (HFTs) features higher levels of repetition frequency, operating voltage, voltage dV/dt, and grid voltage superposition, which challenges the epoxy insulation of HFTs exposed to partial discharges (PDs). Wide ranges of voltage parameters (frequency, voltage, rising edge, and dc bias) and environmental stresses (temperature, humidity, gas, and pressure) are applied to the sphere-gap-insulation-plate structure for PD aging analysis. New patterns are found in this study. For example, the shape of the rising edge affects the lifetime, and the influences of humidity and dc bias are nonmonotonic. Surface charge decay and voltage dV/dt have key effects on the discharge pattern. Early failures with pulse frequencies of 50 and 100 kHz are attributed to the interaction of humidity and electric field. The erosion rate is lower at oxygen-free ambient, while higher with CO2 gas. PD parameters (PD number, PD amplitude, PD delay, PD ignition voltage, and PD frequency) and phase-resolved PD (PRPD) patterns are measured with the ultrahigh-frequency (UHF) method and discussed for all aging conditions. Cumulative PD amplitude and cumulative PD energy have a good correlation to experimental lifetime values. However, due to the transition of the PD regime, the fit model has less explainability on the influence of voltage, humidity, gas, and pressure on the lifetime, which is rarely revealed before.