Differences in partial discharge characteristics due to increased conductivity of the insulation material

This paper presents what influence the choice of PWM level and rise time has on the PD characteristics for two similar wire specimens insulated by different materials and exposed to short rise time voltage waveforms. The test objects were twisted pairs insulated by either polyamide-imid material (PAI) or the more conductive Cr
₂
O
₃
insulation. Here these test objects were fed from either two-, three-, four or five level inverter waveforms of similar type as used in actual applications. Observations are made on the importance of the voltage rise time, the size of the voltage step in relation to the extinction voltages (PDEV) and other observables. To compare the performances, measurements of the PD characteristics were conducted. Specifically, the total number of PDs and their average maximum amplitude per cycle were obtained together with the PRPDA pattern. Previous research suggests that the Cr
₂
O
₃
insulation shows improved degradation properties compared to conventional insulations for sinusoidal waveforms however less difference was observed when exposed to square and PWM waveforms. The experimental results presented in this work showed that the total summed PD magnitude (exposure) drops considerably already when applying a three or higher level inverter for both materials however the PD magnitude remains about the same for both materials tested. Thus to experience the expected advantages with the more conductive insulation a higher level than three appears required. Although in comparison both four and five levels show little influence in number of PDs, the maximum magnitude per cycle is reduced however for both insulation materials. The reduction is however more significant for the Cr
₂
O
₃
insulation. Initial studies suggest that the higher PWM level results in a reduction of the overvoltage level which helps reducing the PD amplitude. As the choice of material influences this phenomenon, the implications is that the advantages with an increased conductivity appears more significant at the higher PWM levels.