Frequency Spectrum Analysis of Partial Discharge Activities in WBG-based AC Machine Drives

High voltage and fast switching speed of wide bandgap ( WBG)-based AC machine drives can aggravate the insulation stress and cause more frequent partial discharge (PD) activities leading to premature failures of the dielectric materials. A particular PD type occurring between the drive system and the ground (e.g., winding to a core and/or winding to a housing) shares an identical leakage current conduction path to that of the conducted common-mode electromagnetic interference (CM EMI) affecting the CM EMI noise level as well. To fully understand and prevent the PD activities in the WBG-based AC machine drives, it is critical to measure and analyze a full frequency spectrum of the PD activities, which are often concurrently measured with CM EMI noise. In the conventional PD measurement systems, the full frequency spectrum of the PD cannot be captured due to an additional high-pass filter (HPF) with a cutoff frequency generally higher than 50 MHz. This HPF is commonly used to segregate the PD signal from the switching and EMI-induced noises, but the PD activities produce electromagnetic noises with a wide frequency spectrum, including less than 20 MHz. In this paper, the full spectra of PD activities during the switching transition of the WBG-based AC machine drive are measured with two different methods without any additional HPF. The measured signals are analyzed in the time and frequency domain to identify major signatures of PD activities in WBG-based AC machine drives.