Flashover characteristics are closely related to the electrical field distribution on the surface of oil-paper insulation. In this study, the effects of electrical field and DC voltage components on the creepage flashover characteristics of oil-paper insulation under combined AC-DC voltage (CADV) are investigated using two electrode models.
Electrode model I is used to study creepage discharge characteristics under a strong parallel electric field component wherein the insulating paper is pressed under the two electrodes. Electrode model II is used to study the creepage discharge characteristics under a strong normal electric field component. The mechanism behind the effect of the electric field and DC voltage components on the creepage discharge characteristics of an OPI under CADV was studied by varying the amplitude of the DC voltage component in the CADV.
The creepage flashover voltage of the OPI under CADV is greatly affected by the electric field component. The damage to the surface of the insulating paper resulting from the creepage flashover is severe under the CADV; the greater the DC voltage component value, more severe is the damage to the surface of the insulating paper. The damage caused by creepage flashover on the surface of the insulating paper under CADV was greater than that under pure AC voltage; the damage in model II was greater than that in model I. The two electrode models have different electric field distribution characteristics under different voltage conditions.
The results demonstrate that when the DC voltage component is large, the flashover voltage under the influence of strong parallel electric field component is approximately 1.2 times higher than that under the influence of strong normal electric field component. When the DC voltage component value increases under the influence of strong normal electric field component, the value of corresponding AC voltage component of the creepage flashover voltage exhibits a decreasing trend. It is believed that due to the strong normal electric field component, the electrons intensify the collision on the surface of the insulating paper. Under the influence of the DC voltage component and due to the presence of space charge on the insulating paper, the distribution shape of carbonized particles on the surface of the insulating paper after creepage flashover is different. Furthermore, the instantaneous energy release during creepage flashover under CADV is higher than that under AC voltage, and the greater the DC voltage component value, more severe is the damage to the fibers on the insulating paper surface.

oil-paper insulation, combined AC-DC voltage, creepage discharge, electrical field component, flashover voltage, space charge