Degradation characteristics of insulation near aluminium foil edges inside dry‐type bushing cores under electrothermal compound stress

Insulation breakdown of dry-type valve-side bushing cores under electrothermal compound stress has become a key factor limiting the safe and stable operation converter transformer. Based on slices real core, it is observed for first time that fold, peeling off, fracture defects exist near aluminium (Al) foil edge, accompanied by many metal spikes with curvature radius <4 μm. Coupled capacitor screen thickness ∼2 mm, an electrical tree model designed to simulate insulation edges inside cores, whose degradation characteristics are investigated accordingly. The results show high temperature significantly increases dielectric loss heat energy density Joule density, accelerating Al edges. parallel crepe paper interface accelerates growth trees AC-superimposed positive DC voltage, while this effect not significant AC voltage. As consists wooden fibres 10 μm diameter, tend grow along interfaces between epoxy resin form “feathery” in resin-impregnated samples.