AC power-loss measurements in solid dielectric materials at industrial frequency

In the standard specifications of AC dielectric-characteristic measurements of insulating materials, the test voltage is prescribed as approximately sinusoidal. However, a limit is given for the highest deviation of the HV waveform from the correct sinusoidal shape in terms of a ¿¿ 5% tolerance range of the crest factor value respect to the sinusoidal waveform. In the paper, the errors in solid dielectric losses, evaluated at industrial frequencies, due to an applied voltage distorted by a 3rd-harmonic are studied. The analysis is carried out by comparing the dielectric losses in the distorted steady-state with those occurring under the sinusoidal steady state on the basis of the same peak value or of the same RMS value of voltages, thus establishing a reference with the usual methods adopted in measuring test voltage as required by the HV test technique. Curves are obtained showing the errors made in dielectric-loss evaluation as function of 3rd-harmonic content in relation to the 3rd-harmonic phase angle. It is emphasised that crest factor does not completely account for errors in power-loss evaluation. The theoretical results are then shown to be in very good agreement with the experimental data obtained in testing some solid dielectric materials.