Effect of oil-paper damped state on characteristics of surface discharge process under AC-DC combined voltage

Highly damped oil-paper insulation system is respected as the predominant factor of surface discharge phenomena in converter transformers, which consist AC and DC combined electric field. The present study focuses on the surface discharge characteristics of oil-paper insulation system with different moisture content under AC-DC combined voltages. AC and DC voltages were applied to the sphere-plane defect model in the established experimental platform, and various oil-immersed pressboards with different moisture content were prepared according to the cases in actual converter transformers. The developing processes from the initiation of surface discharge until final flashover were observed for each moisture content and DC component. PD parameters and patterns were acquired by detector using pulse current method. Results show that moisture has a significant impact on surface discharge activities in oil-paper insulation. It is shown that the inception and flashover voltage decrease with the increase of moisture content, while the amount and amplitude of discharge increase. As the damped state in oil-impregnated paper samples gets more serious, the carbon trace on the surface of pressboard is much more obvious. The process of the whole surface discharge under AC-DC combined voltage can be divided into three stages. The findings of the current study are useful in further research on fault diagnose in converter transformers.