High-fundamental-frequency modulation for the DC-DC modular multilevel converter (MMC) with low switching frequency and predicted-based voltage balance strategy

This paper proposes the high-fundamental-frequency modulation of the dc-dc MMC-transformer-MMC topology, which is a potential candidate for the HVDC and transportation applications. With the proposed modulation, the MMC arms generate a high-fundamental-frequency ac voltage applying on the transformer, the size of the transformer, as well as the inductors and capacitors of the MMC arms, can be reduced. On the other hand, the proposed modulation ensures all the MMC submodules strictly working at the same fundamental frequency, and the switching losses of all the submodules are approximately balanced. Besides, the capacitor voltage balance strategy under the proposed modulation strategy is presented, and the current stresses of the submodules are also analyzed. Finally, the proposed modulation method is verified by a 10 kV/20 kV/1 MW simulation.