Influence on PD Parameters due to Voltage Conducted Disturbances

In the standard specification of ac dielectric-characteristic measurements of insulating materials, test voltage is prescribed as "approximately sinusoidal" when the highest acceptable deviation of the HV waveform, from the correct sinusoidal shape, is limited to a ±5% tolerance range of the crest factor value. In the field of partial discharge (PD) measurements and their statistical data processing, on which forecasts of long term behavior of components and their reliability are currently carried out, the results of elaborations depend on the voltage wave shape. In this paper, the errors in PD measurements, evaluated at industrial frequencies, due to applied voltages distorted by 3rd and 5th harmonics, are studied. The analysis is carried out by comparing the main characteristic parameters of PD, in the distorted steady state, with those occurring under the sinusoidal steady state. This is obtained on the basis of the same RMS value of voltages, thus establishing a reference with the usual methods adopted in measuring test voltage as required by prescribed HV test technique. Stochastic and statistical analysis and parameter's curves, applied to a few of possible PD markers, show the differences in the measurements, when these are obtained as function of harmonic content percent and phase angle. Moreover it is emphasised that crest factor does not completely account for errors in PD pattern evaluation.