Endurance of Epoxy Resin Cast Insulation With Interfacial Micro‐Crack Defect Under 50 Hz Sine and Kilohertz Square Voltages

Crack‐type defects are commonly found in epoxy composite cast insulation due to residual, thermal and external stresses. When combined with inverter‐type voltage stress, as in epoxy cast high‐frequency transformers (HFTs), the likelihood of insulation failure increases significantly. In this study, a method setting microcracks perpendicular to the metal‐insulation interface was proposed on the cast insulation samples, and the endurance of epoxy insulation under high‐frequency square wave voltage was found to be significantly lower than that under power‐frequency voltage. A threshold electric field effect on failure characteristics was identified with values of 4 /mm for 50 Hz sinusoidal and 2 /mm for 1∼50 kHz bipolar square wave voltage, which are lower than previously reported results. This frequency dependence is attributed to differences in the electric field distribution and discharge propagation mechanism inside the crack. Theoretical analysis of internal direct breakdown within crack defects was performed, showing good agreement with experimental results. The lowest crack breakdown voltages were yielded under conditions of thermal decomposition gas and low pressure. Finally, a relationship between permissible electric field and crack length was derived to guide insulation design that avoids rupture‐type electrical failure of solid insulation triggered by direct internal breakdown of crack defects.