A General Measurement Set-up Approach to Evaluate Insulation System Quality by Exploring PWM and DC biased waveforms

Abstract

This article presents a novel test approach able to detect and evaluate PD characteristics when employing various wave shapes similar or close to what is used in different applications. The method covers the range from bipolar PWM with ultra-fast rise times to dc voltages with or without superimposed high -frequency content. Measurement results from tests on twisted pair aimed to resemble a random wound motor are presented. In particular, the influence of conductivity, permittivity, and rise time is explored by applying square- shaped waveforms. The PDs appearing after the unipolar flanks are compared with full -size bipolar voltage steps. One important observation presented is that when the zero voltage was reached and kept, the remaining charges within and surrounding the defect produced a PD pattern (PD echo) which resembles the charge amount present within the winding. Additionally, PWM shaped voltage waveforms superimposed on dc voltages present advantages when further evaluating the influence of material properties. Different step sizes are employed to represent dc voltage with or without superimposed voltage ripple to elucidate the influence on PD characteristics also on HVdc like applications. It can be concluded that higher frequency harmonics will result in considerably larger PD exposure. This article shows that during material evaluation, important common properties and guidelines can be found between these different waveform extremes, suggesting that a more efficient insulation coordination can be obtained selecting the most appropriate or time- efficient evaluation method.