Insulation degradation and fracture cracks of mechanical impacted thermal‐ageing oil‐impregnated papers

Abstract

In order to elucidate mechanical impacts affecting the insulation strength of thermal-ageing oil-impregnated pressboards, we perform mechanical degradation experiments of impacting forces that are represented as the winding-suffered stresses on thermal-ageing oil-paper insulation in transient short-circuit of transformer. Water content, degree of polymerisation (DP), partial discharge inception voltage and AC breakdown field strength are tested and analysed for diverse thermal-ageing levels. An analytical model of mechanical degradation is established by relating dielectric breakdown strength to mechanical impact. Mechanical impacts result in less water content in thermal-aging oil-impregnated pressboards while causing reduction and increment of DP, respectively, in mildly and severely aged oil-impregnated pressboards. Partial discharge inception voltage and dielectric breakdown strength vary significantly with the numbers of impact cycles, showing a maximum value under low-intensity impact and decreasing monotonously under high-intensity impact. High-intensity mechanical impacts will evidently exacerbate the thermal deterioration process of oil-impregnated pressboards, which is primarily attributed to cellulose fibre fractures in pressboard cracks produced by equivalent tensile stress of mechanical impacts. It is acceptable for mildly aged oil-impregnated pressboards to suffer low-intensity mechanical impacts that will alleviate insulation degradation by restraining partial discharge in smaller oil-gaps and prolonging breakdown discharge pathways along impact-caused cracks.