The effect of DC superimposed AC voltage on partial discharges in dielectric bounded cavities

Voltage source converters is used in HVDC stations in offshore HVDC transmission systems, between the AC and DC power grid. The AC ripple voltage on the DC side of the HVDC stations can be in the range of 1-10 % of the nominal DC voltage, depending on the size of the filter employed. For offshore HVDC grids, there is a drive to use polymeric insulated cables on the DC side. This work investigates how an AC voltage at power frequency superimposed on DC voltage influence the partial discharge magnitude and repetition rate in artificial cylindrical cavities in polymeric insulation. The AC voltage is kept below the AC partial discharge extinction voltage, and the DC voltage is kept above the DC partial discharge inception voltage. A resistor-capacitor ABC-circuit model is used for prediction of partial discharge magnitude and repetition rate under combined AC and DC voltage. Measurements has been performed on a test object of 3 layers of PET film with 1 mm radius cylindrical cavity in the middle layer. The results indicate that an AC voltage ripple with an amplitude lower than the AC partial discharge extinction voltage will increase the number of large discharges, compared to a DC voltage without ripple, but the repetition rate will be several orders lower than the AC voltage frequency.