Electrical tree growth under harmonic frequencies characterized by partial discharges waveforms

In this paper we address the problem of determining the stage of growth of electrical trees from the evolution of the PD pulse waveform during electrical tree propagation. Five parameters that characterize PD pulse waveform were evaluated in their capability to assess the stage of growth of electrical trees by observing their evolution and variation with tree growth. Electrical trees were grown in epoxy resin using needle-to-plane configuration and applying 12 kV AC and frequencies 50-550 Hz. PD pulse waveforms at different stages of tree growth were captured using a high frequency current transformer, and the tree length monitored by an optical camera to correlate it with PD behavior. The results showed that there were changes PD pulse waveform during electrical tree propagation, especially a variation in the decay part of PD pulses with tree growth. In particular, the parameters were able to pre-identify tree crossing of the insulation gap and breakdown.