Dielectric Behavior of Solid/Liquid Insulation System

Dielectric loss of the PP/oil system was decreased by high voltage application beofe the dielectric measurement. The extent of decrease depends on the magnitude and the duration of the prestress voltage. This decrease in dielectric loss is caused by trapping of the impurities on the interface of the PP/oil. We found that the dielectric loss tends to go back up to the initial value with time after the voltage has been removed. The experimental results can be interpreted in terms of trapping of ions if the following process are assumed: in the very low voltage region, ionic carriers are accumulated near the interface without trapping; and as the voltage increases, they are trapped near the interface, causing a decrease in ionic concentration in the thin oil layer.