Thermal Analysis of Modular Multilevel Converters Under Subsynchronous Oscillation

Subsynchronous oscillation (SSO) may occur in modular multilevel converter based high-voltage direct-current (MMC-HVDC) system, leading to overcurrent and overheating of power devices in MMCs, thus threatening the reliability of MMCs. As the mechanism of SSO in MMC-HVDC has not been clarified, it is essential to investigate the thermal performance of power devices to promote the reliability of MMCs. In this paper, junction temperature is calculated by the power loss model and thermal network. Then, the relationship between junction temperature and SSO frequency is investigated. It is concluded that with the increase of SSO frequency, both the maximum junction temperature and thermal cycling go down firstly and then go high, and both are lowest when SSO frequency is 25Hz. Further investigation reveals that the low frequency loss fluctuation caused by SSO is the main cause to this thermal phenomenon. Finally, a simulation model and an MMC prototype with six sub-modules per arm are established to verify the theoretical analysis.