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Modeling of partial discharges in the presence of harmonics

Harmonics are frequency-domain representations of time-domain occurrences which may increase the peak, rms and rate of change of the applied voltage. Small variations in harmonic content can result in significant changes to measured partial discharge patterns. This experimental study sought to model partial discharges due to electrical treeing as a consequence of varied harmonic content reducing the power quality. The total harmonic distortion (THD) and waveshape (Ks) indices were varied to a maximum of 40% and 1.6 respectively. An unprecedented six harmonic orders (5th, 7th, 11th, 13th, 23rd and 25th) have been simultaneously superposed onto the fundamental frequency (50 Hz). Electrical trees were developed in a point-plane geometry employing 2 μιη radius hypodermic needles and a 3 mm gap in epoxy resin (LY/HY 5052) samples at constant voltages of 8.4 kV peak and 14.4 kV peak. Modeling partial discharge activity with the function cosh (V + dV ÷ dt) provided a good fit for identifying peaks and periods of no partial discharge activity ('dead zones') on the phase-resolved partial discharge plots. Waveforms with increased magnitude of the 7th harmonic only produced anomalous results which did not fit the function.