Over-Modulation Operation of Hybrid Modular Multilevel Converter with Reduced Energy Storage Requirement

The hybrid modular multilevel converter (MMC) has the advantages of DC fault blocking and over-modulation capability as the promising candidate in future multi-port HVDC systems. Especially, the over-modulation ability could increase the modulation index higher than 1 without voltage distortions on the AC side. However, the hybrid MMC under the over-modulation operations could suffer from the voltage deviations between the half-bridge submodules (HBSM) and full-bridge submodules (FBSM) capacitors. In this paper, the structure of the self-balancing branches (SBB) is introduced into the hybrid MMC to eliminate the aforementioned voltage deviations. As a result, the energy storage requirement (ESR) of the hybrid MMC under the over-modulation operations could be greatly reduced compared with the conventional hybrid MMC without SBBs. Besides, the voltage balancing strategy is improved to avoid the impulse current in the conduction path of the SBB, by which the current stress of the power devices could be decreased significantly. The simulations and experiments are carried out to verify the effectiveness of the proposed hybrid MMC. It shows that the ESR of the hybrid MMC could be reduced by 30% with decreased power losses under the over-modulation index 1.5 compared with the conventional hybrid MMC.