Effect of high voltage impulses on partial discharge characteristics of oil-impregnated paper for online diagnostics

The onset of Partial Discharge (PD) activity within an insulation system is often an important signature of degradation. Changes in the PD pattern aid assessment of the insulation degradation level. High voltage transients such as lightning and switching impulses are inevitable phenomena in power systems. They can impose electrical stresses on the insulation system of power components such as power transformers and their bushings which may lead to their failure. After the arrival of a transient the AC PD pattern may change, in a way that is influenced by the type of PD source. In this paper, the effect of HV impulses on the change of surface and cavity discharges of oil-impregnated paper has been investigated. With resemblance to the real situation, insulation samples have been subjected to an impulse voltage superimposed on the AC voltage. The behavior of PDs before and after the impulse for different defects and moisture content levels in oil-impregnated paper has been studied. 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. Recording the voltage signals and associated PD measurements during and after the incident of high voltage transients can be used for assessing the insulation condition of power transformers and their bushings and interpretation of the recorded signals can be used for online diagnostics.