Partial Discharge Features for Power Electronic Transformers Under High-Frequency Pulse Voltage

Power electronic transformers (PETs) are becoming popular in the modern dc distribution grid due to their high efficiency and flexibility. To increase the power density of PETs, high-switching frequency would be adopted for the power switches. However, this may eventually increase the risk of partial discharge (PD) as the internal insulation space is reduced. In order to investigate PD characteristics in PETs under high-frequency pulse voltage, an experimental platform was set up in the laboratory, in which a twisted pair of wires model was used to test the insulation failure between windings in PETs. The PD behaviors were thoroughly investigated in the frequency range of 50 Hz to 20 kHz by time-resolved PD (TRPD). The results reveal that at high frequency, both the initial discharge voltage and discharge repetition would increase, while the PD amplitude decreases. Moreover, the PD pulses mainly occur at the field strength inversion of the rising and falling edges, the discharge time width tends to increase at high frequency. By analyzing the discharge model under pulse voltage, the discharge amplitude decrease and discharge times increase due to the difference of charge accumulation field intensity and effective discharge time at different frequencies. Under high-frequency voltage, the number of PD and the proportion of discharge time is much higher than 50 Hz, which may be the main reason for high-frequency insulation failure prematurely. According to the insulation life model based on the total discharge amplitude, the insulation life of 20 kHz is slightly higher than 10 kHz. When designing HFT, 20 kHz should be elected as operation frequency rather than 10 kHz.