Effect of superimposed impulses on AC partial discharge characteristics of oil-impregnated paper

The effect of HV impulses on surface and cavity partial discharges (PD) of oil-impregnated paper (OIP) is investigated in this work, by subjecting specimens to impulse voltages superimposed on a continuous AC voltage. The superposition of transient and AC voltage is relevant to insulation systems in power-system equipment, which is exposed to transients such as lightning and switching impulses during its operation. For several different PD-generating defects in oil-impregnated paper, the PD behaviors before and after the impulse are compared. The influence of moisture content is studied by using OIP with moisture content <;0.5%, 3.0% and 5.5%. The results show a distinctive behavior of PD due to the impulse for each defect and insulation condition, either by change in the PD rate and magnitude or by causing PD inception or extinction. Recordings of the voltage signals and associated PD measurements during and after the incidence of a high voltage transient are of potential use for condition assessment of the insulation of power transformers and their bushings, as a form of online diagnostics.