Comparison of Charge Mobility in Insulating Oil and Oil-immersed Paper

Oil-paper insulation is the main insulation of converter transformers. To understand the charge transportation process in oil-paper insulation, the charge mobility in insulating oil and oil-immersed paper is measured based on PEA method. The positive charge mobility of insulating oil is dozens of times higher than that of oil-immersed paper, and the negative charge mobility of insulating oil is hundreds of times higher than that of oil-immersed paper, wherefore a large amount of charge accumulates at oil-paper interface under DC voltage. The SEM images show that the fiber of the ordinary insulating paper sample has more defects than the heat-resistant insulating paper sample, resulting in more deep traps distributed in the body and surface, which not only causes the lower charge mobility, but also makes it more difficult for the trapped charges at oil-paper interface to escape. Both of the charge mobility and the surface state of oil-immersed paper have an important influence on the charge transport at oil-paper interface. Moderately improving the charge mobility in oil-immersed paper and reducing the fiber defects of insulating paper are beneficial to suppress the charge accumulation at oil-paper interface.