A Novel Modular Multilevel Converter With Ripple Current Elimination Channels Based on Isolated CLLC Resonant Circuits

This paper proposes a novel multilevel topology 'Modular-Isolated-Multilevel-Converter' which achieves almost zero low frequency capacitor voltage fluctuation. It inherits the structure of MMC but replaces the half bridge module by the newly proposed Isolated Half-Bridge (IHB). The fundamental and 2nd order harmonic frequency current originally in the MMC module capacitor have been eliminated through connecting the secondary sides of the IHB at the same level of the three phases together. The elimination is due to that the 1st and 2nd order components in the arm current are 120° phase shifted in three phases. Therefore, the module capacitance is reduced by more than 10 times since it only carries switching frequency ripple. Moreover, the arm inductance can also be significantly reduced since the 2nd order harmonic current disappears. The topology is specifically suitable for variable frequency drive application, because its capacitance and inductance are not affected by the output frequency. The challenges of zero frequency start-up when MMC is adopted for VSD can be addressed here. In the paper, the operation principle of the proposed MIMC is fully analyzed and the mathematical model is built. Moreover, a methodology of capacitor sizing and arm inductor design for general MMC topology is proposed. The detailed design considerations for MIMC are also discussed and presented. The plant modeling and control strategy have been proposed for MIMC. A 55-kW simulation is carried out to verify the theoretical analysis. And a 6-kW downscaled hardware prototype is also developed to demonstrate the benefits of the new topology over the traditional MMC.