Partial Discharge Characteristics of Polyimide Under the High-Frequency Multilevel Stepped Waveform

High-power high-voltage high-frequency transformers (H3Ts) have been widely applied in the modular multilevel converter (MMC) due to their high efficiency and flexibility. However, the special electric stress for the insulation is introduced into this type of power converter, that is the high-frequency multilevel stepped waveform stresses generated by the converter’s switching actions. Therefore, it is important to investigate partial discharge (PD) characteristics in H3Ts under the stepped waveform. This article analyses the influence of frequency and voltage level numbers on the PD initial voltage (PDIV) and time-resolved PD (TRPD). Experimental results indicate that PDIV shows a decreasing trend within the frequency range of 10–100 kHz. Moreover, it is observed that the five-level stepped waveform results in a 50% reduction in PD numbers at 10 kHz when compared to the two-level stepped waveform. The space charge movement during platform time induces the production of PD, which is shown as two peaks on the TRPD pattern. These results provide experimental and theoretical support for the diagnosis of PD under MMC stepped waveforms for the application in H3Ts.