AC Conduction in Impregnated Polymer Insulating Systems

Experimental data are presented describing the relationship between the dissipation factor of capacitors containing polypropylene film impregnated with trichlorodiphenyl and the applied voltage, the duration of the applied voltage, and the thickness of the liquid layer adjacent to the polymer film. A general theoretical treatment based on the equivalent circuit representation of a low-loss polymer capacitor in series with a high-loss liquid capacitor provides a qualitative explanation of the experimental data. The results demonstrate that ion conduction in the thin liquid layers between the impregnated polymer and the electrodes can account for the observed dissipation factor. The ion transit time across the liquid layer is very short compared to the period of a half-cycle so the ions are immobilized on the film or foil surfaces for a large fraction of each half-cycle. The ion transit time and consequently the dissipation factor are strong functions of the applied field and the thickness of the liquid layer. These factors as well as a trapping of the ions by the film causing the dissipation factor to decrease with time on voltage must be considered in interpreting the dissipation factor data of impregnated polymers and in attaining the potentially low losses possible in these systems.