Increased breakdown voltage of oil impregnated direct current filter capacitor

The increase of the electric stress, in large High Voltage direct current (DC) filter capacitors manufactured with all polypropylene film dielectric impregnated with synthetic hydrocarbons, is limited due to the high dispersion of the values of the DC breakdown voltage. So, the nominal voltage (Un) have to be taken at low values, in order to avoid a too high failure rate at the routine DC test voltage (equal to 2 times Un). The situation is more favourable when AC stresses are applied, because the dispersion of the values of the AC breakdown voltage is lower than the DC ones. Breakdown stress curves for all polypropylene film elements impregnated with DT/DBT liquid both AC and DC are shown. The paper describes the results obtained adding to the impregnating liquid a scavenger. The DC breakdown voltage dispersion is reduced and corresponding results on routine DC test voltage on complete units are shown. Due to this discover the capacitor dielectric stress may be increased. The result is interesting because for some applications the reduction in volume and mass of DC capacitors is strongly required.