An Investigation of the Harmonic Impact on Electrical Treeing Partial Discharge Activity

Partial discharge activity is both a cause and symptom of electrical ageing mechanisms induced by electrical stress. This experimental study sought to differentiate variations in partial discharge patterns in the presence of harmonics and due to changes in the state of an electrical tree. Harmonic content has been superposed with the power frequency (50 Hz) creating six distorted, composite waveforms. The Total Harmonic Distortion (THD) and waveshape (K-s) indices were varied to a maximum of 40% and 1.6 respectively. Using each composite waveform as well as the fundamental, partial discharge activity was sequentially captured from electrical trees developed in pin-plane geometry epoxy resin (LY/HY 5052) samples defined by a 2 mm gap. Hypodermic needles of 2 mu m radius and a test voltage of 10.4 or 14.4 kV peak were employed. The results illustrate distorted waveforms influence the partial discharge pattern which can erroneously be interpreted as a change in the dominant ageing mechanism and vice versa.