Comparison of Dielectric Loss Measuring Methods on Epoxy Samples under Harmonic Distorted Voltages

Abstract:
The increasing number of power electronic devices in medium voltage networks lead to a harmonic distortion of the voltage waveforms in electrical systems. Medium frequency components up to several kHz are superimposed on the fundamental grid voltage. The distorted voltage waveforms stresses the insulation systems due to the additional current and hence additional dielectric losses. The heat generated from dielectric losses is suspected to be an accelerator to insulation ageing below partial discharge inception. Therefore, the systematic assessment of the losses and their correct quantification is crucial to designing reliable insulation systems with a high lifetime. The investigation of losses under nonsinusoidal and distorted voltage waveforms is not trivial and has been subject to little research to date, compared to investigations at 50 or 60 Hz. In this paper, several approaches of quantifying dielectric losses under multifrequent voltage waveforms are presented and compared. The losses of cast resin specimens are quantified with a test setup using a reference measurement method. The loss computation algorithms range from previously utilised Extrapolation and Superposition approaches to a direct power measurement technique. The results presented in this contribution show that the different measurement procedures provide considerably different outcomes for which possible explanations are given. The presented data indicates that realistic field strength conditions should be ensured in order to quantify the losses under distorted voltage waveformes correctly.