Investigation of moisture influence on dielectric response of ester oil impregnated pressboard

Demand for alternative insulating liquids for power transformers are increasing due to the safety and environment concerns. Ester based insulating liquid is considered as a potential alternative for mineral oil since it is highly biodegradable and has high fire point. On the other hand, Polarization and Depolarization Current (PDC) measurement technique has been widely used to evaluate moisture and ageing condition of transformer´s insulation system. However, there is limited experience of using PDC measurement for ester liquid filled power transformers. This paper investigates dielectric response of four types of insulating liquids impregnated pressboard specimens under varying moisture and temperature conditions. The PDC measurement results of esters and mineral oil impregnated pressboards at approximately same moisture levels are compared in the paper. Moreover, this paper adopts an equivalent circuit for interpreting the PDC response of the pressboard specimens. The variations of the parameters of the equivalent circuit under different moisture levels are also discussed in the paper.