Creepage discharge phenomenon has a close relationship with the electrical field distribution on the surface of oil-impregnated pressboard（OIP）. Thus

we investigate the effects of electrical field component on characteristics of creepage discharge in OIP under combined AC/DC voltage by two electrode configurations. The experimental results show that

for both electrode configurations

there is the lowest under AC voltage

and the flashover voltage increases with DC component of combined AC-DC voltage. Besides

there is no polarity effect in the absence of back electrode configuration

but the polarity effect exits in the presence of back electrode configuration. Furthermore

the flashover voltage under the negative DC component is lower than that under the positive DC component because of higher electronegativity of hydroxyl in the cellulose of OIP. The flashover voltage in the absence of back electrode configuration is approximately 1.5 times that in the presence of back electrode configuration. However

the carriers will enhance collisions with the surface of OIP under higher vertical electrical field component. When the vertical electrical field component is higher

the creepage discharge phenomenon easily happens under a lower AC voltage component stress in the space charge balance condition in OIP. The flashover voltage decreases with a higher vertical electrical field component. When voltage is near the peaks and valleys of AC voltage component

the creepage discharge phenomenon happens.