Investigation on the Effect of Insulation Material Formulation and Coating Thickness on the Insulation Performance of Epoxy-Metal Interface in Sandwich Structure

The epoxy-metal interface is the weakest part in the internal insulation of epoxy insulation parts. Improving the insulation performance of the epoxy-metal interface through material formulation and process modification is an effective way to reduce insulation accidents. In this paper, we investigate the influence of epoxy formulation, semiconductive coating, and surface states of metal inserts on the insulation performance of epoxy-metal interfaces. First, post insulators with four epoxy formulations, two insert electrode surface states, and three thicknesses of semiconductive adhesive coatings were set as samples. Second, the partial discharge characteristics and breakdown voltage of the insulator samples were measured on a non-partial discharge power frequency experimental platform, using the step boost method. According to the calculation results of electrostatic field distribution based on finite element analysis, the maximum electric field strength at the epoxy-metal interface increases with the increasing of the semiconductive coating thickness, but is almost unaffected by its relative permittivity. There is generally no obvious partial discharge signal before internal breakdown of epoxy post insulator without interface defects. Compared with smooth metal inserts, the insulators with knurled metal inserts have a higher breakdown voltage. Besides, the samples with the highest breakdown voltage did not use the epoxy formulation with the highest breakdown electric field strength, which is limited by the compatibility among the epoxy resin material, the semiconductive coating, and the metal insert. The results suggest that in order to improve the insulation strength of the epoxy-metal interface, it is necessary not only to adopt knurling process on the surface of the metal inserts, but also to consider the compatibility among the three materials when selecting the epoxy insulation material and the semiconductor coating.