Multilevel PWM: A Tool to Explore Insulation PD Characteristics

The benefits of employing multilevel pulsewidth-modulated (PWM) waveforms for diagnostic purposes compared to sinusoidal- and square-shaped voltages are presented in this paper. Examples on how the high voltage circuits are designed to enable these studies, as well as results, are discussed. To demonstrate the approach, it is investigated what influence the choice of PWM levels has on the partial discharge (PD) characteristics for two similar wire specimens insulated by different materials. Here, to resemble a motor winding, two different twisted pair test objects insulated by either pure polyamide-imid (PAI) material or with added conductive Cr₂O₃ material were exposed to voltages with ultrashort rise times. The test objects were fed from either two-, three-, five-, or seven-level inverter waveforms of similar shape as in actual applications. To compare the performances, measurements of the PD characteristics were conducted. Specifically, the total number of PDs, as well as their average maximum amplitude per cycle, was obtained together with the phase-resolved PD pattern for each case. The experimental results presented in this paper show that the total peak and summed PD magnitude (exposure) drop considerably more for the Cr₂O₃ insulation compared to PAI when applying a five-level or higher level inverter. This reduction is more significant for the insulation with added Cr₂O₃ material and the usage of higher inverter levels reveals this. As more information can be obtained using higher inverter levels, the evaluation of new material constitutions is preferably done utilizing at least a three-level inverter or higher