Capacitance Reduction Method on Hybrid MMC Topology with High Modulation Ratio

Hybrid MMC (Hy-MMC) has broad application prospects because of the negative level output ability in its bridge arm. A Hy-MMC topology with a high-modulation ratio is designed in this paper. A second-harmonic current injection (SHCI) strategy based on Hy-MMC in high-modulation operation is also proposed to improve the power density of Hy-MMC effectively in steady-state operation. The amplitude of SHCI is determined from the perspective of the root mean square (RMS) value of bridge arm current, and the optimal initial phase angle is determined from the multi-objective optimization (capacitor voltage ripple of sub-modules (SMs), power loss, and peak value of bridge arm current). The effectiveness and engineering practicability of the proposed SHCI strategy based on Hy-MMC in high-modulation operation is verified by the electromagnetic transient (EMT) simulation using PSCAD/EMTDC@. The simulation results show that the capacitor voltage ripple of SMs can be effectively decreased by 61.98% or the capacitance can be decreased by 40%. The power loss is also analyzed. In addition, little influence of low capacitance on multi-operation conditions of Hy-MMC is verified by EMT simulation.