A selection principle of submodule switching state vectors for switching frequency reduction in voltage self-balancing half-bridge modular multilevel converters

The problem of submodule switching frequency reduction in half-bridge modular multilevel converters (HB-MMCs) with capacitor voltage self-balancing control is considered and explored in this paper. A selection principle of submodule switching state vectors is proposed based on the voltage self-balancing switching state matrix, aiming to lower submodule switching frequency and device losses. The relationship between system stability and submodule switching signals is revealed according to the capacitor voltage self-balancing characteristics, and the full-rank constraints on the voltage self-balancing switching state matrix are proposed. Considering the tradeoff between switching loss and capacitor voltage fluctuation, the evaluation indexes of voltage self-balancing control effect are determined. The selection principle of submodule switching state vectors and the optimized construction method of switching state matrix are presented. Voltage self-balancing HB-MMC models are built in MATLAB/Simulink, and it is verified that the submodule switching state vector selection principle proposed in this study can effectively reduce switching frequency while meeting the evaluation requirements of practical engineering projects, so as to achieve the balance between switching loss and steady-state capacitor voltage fluctuation. In this work, the authors propose a selection principle of submodule switching state vectors based on the voltage self-balancing switching state matrix with lowered submodule switching frequency and device losses. The evaluation indexes of the voltage self-balancing control effect are determined, and the optimized construction method of switching state matrix is presented. The selection principle proposed can effectively reduce switching frequency while meeting the evaluation requirements of practical engineering projects, so as to achieve the balance between switching loss and steady-state capacitor voltage fluctuation.image