Overmodulation 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 overmodulation capability as the promising candidate in future multiport HVdc systems. Especially, the overmodulation ability could increase the modulation index higher than 1 without voltage distortions on the ac side. However, the hybrid MMC under the overmodulation operations could suffer from the voltage deviations between the half-bridge submodule (HBSM) and full-bridge submodule (FBSM) capacitors. In this article, the structure of the self-balancing branches (SBBs) 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 overmodulation 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 overmodulation index 1.5 compared with the conventional hybrid MMC.