Surface flashover development on transformer oil-pressboard interface

Highly insulating pressboard immersed in transformer oil applies a polarization force on charge carrying streamers, proportional to the permittivity difference between the oil and the pressboard. If the pressboard permittivity is greater than the oil permittivity, the attractive force turns the streamer into a surface flashover. On the other hand, a low permittivity pressboard interface repels the approaching streamer and reduces the streamer velocity. Theoretical analysis of electric field propagation through the interfacial surfaces, oriented in either parallel with or perpendicular to the streamer propagation direction, is presented. Pressboard relative permittivities of 1.1 and 4.4 have been studied while the oil relative permittivity is assumed 2.2.