Thermal Optimization of Modular Multilevel Converters With Surplus Submodule Active-Bypass Plus Neutral-Point-Shift Scheme Under Unbalanced Grid Conditions

The thermal design of highly reliable modular multilevel converters (MMCs) is significant for the voltage-source converter based high-voltage direct current (VSC-HVdc) systems, especially under the ac unbalanced fault. In this paper, the surplus submodule is employed as new control freedom to optimize the thermal behavior of the MMCs, because its number is linearly increased with the ac voltage dip. With the surplus submodule active bypassed (SSAB), the total submodules of the MMC in one arm can be taken turns to reduce the thermal stress of the hottest power device. In order to obtain equal ac voltages in three phases to improve the SSAB strategy, the neutral-point-shift (NPS) scheme is introduced under the unbalanced conditions, especially single-phase-to-ground fault. Through the balancing of the amplitude of the MMC ac voltages with the NPS scheme, the thermal stress asymmetry of theMMC under the single-phase-to-ground fault is mitigated, and the junction temperature of the most stressed power devices is significantly reduced. Finally, the simulation and experiment results show that the junction temperature rise of the most stressed power device is reduced by nearly 30% with the SSAB plus NPS scheme.