An Online Monitoring Scheme for Partial Discharge Signal of Sub-module IGBTS Used in MMC

Because of the merits of no reactive power compensation and no commutation failure problem, Modular Multilevel Converter (MMC) technology is being applied in many scenarios, such as multi-terminal HVDC, offshore power transmission, and DC power distribution network. The status of IGBTs is critical to the MMC-HVDC system, and their number and value are very large. Therefore, it is very important to monitor the health status of IGBTs on-site. On the other hand, the maximum blocking voltage of IGBT modules is up to 6.5kV and the current reaches more than 2000A. Increased blocking voltage of IGBT can cause the local electric field in the device becomes large enough to induce partial discharge. In this paper, an online monitoring scheme for the partial discharge signal of the IGBT is proposed. The partial discharge signal can be detected from the current waveform of the submodule and the IGBT containing defects could be located. To validate the proposed scheme, a simulation model of single-phase seven-level MMC circuit was built and an IGBT containing defects was set up to discharge when its blocking voltage exceed a certain value. The result shows that the scheme is very sensitive to partial discharge signal and can recognize the partial discharge signal accurately even if it is weak.