Effect of Interfacial Contact Morphology on High-Frequency Breakdown Characteristics of Epoxy Resin-polyimide

Solid-state transformers are easy to introduce gas cavities in the manufacturing and assembly processes, which is considered to be one of the reasons for the decrease of interfacial breakdown strength of epoxy resin-polyimide (EP-PI) composite insulation. Based on the contact theory, it is pointed out that the size and shape of the cavity are related to the contact pressure and surface roughness. Combined with the finite element simulation, it is found that the gas cavity will cause electric field distortion at the interface. The effect of interfacial contact morphology on discharge characteristics was investigated by building a platform for the high-frequency interface discharge test. Experimental results show that the interfacial breakdown voltage is affected by surface roughness and contact pressure. The interface breakdown voltage increased from 12.91 kV to 19.34 kV when the contact pressure was increased from 5 bar to 20 bar after sanding with a sandpaper grit of 2000, an increase of 49.81%. The smoother the surface of the two materials, the higher the breakdown voltage at the interface. The mechanisms of surface roughness and contact pressure on the discharge characteristics of the EP-PI interface are further investigated. Surface roughness hinders the development of secondary electron avalanche by affecting the actual incident angle of electrons and reducing the kinetic energy of electron acceleration. At the same time, when the surface roughness increases, the surface resistivity decreases, and the electron avalanche current increases. The synergistic effect of the two factors promotes the development of interfacial discharge. While the contact pressure will directly affect the proportion of interface contact area and the density of contact asperity, and introduce more interfacial discharge obstacles, thereby increasing the breakdown voltage at the interface. The above research results provide a basis for the optimization design of a solid-state transformer composite insulation system.