Optimal Arm Current Reallocation of Modular Multilevel Matrix Converter Dedicated for Power Grid Interconnection

The modular multilevel matrix converter (M3C) is the high-power three-phase to three-phase AC/AC converter of the next generation. However, when the M3C is applied for power grid interconnection, with equal or close input and output frequency, the current distribution in its nine arms becomes significantly uneven. Furthermore, the current distribution pattern is directly influenced by the randomly changing input-output angle-difference, forcing each arm to be designed based on the highest-possible current, which significantly increases the required semiconductor rating and converter cost. To address this issue, a novel current reallocation is proposed to attenuate the highest-possible arm current of M3C. The equal-frequency operation of M3C is first modeled and analyzed, and the operation sweet-spot where each arm stays in natural energy balance is located. Then, the current reallocation is analytically derived for sweet-spot operation. By injecting proper circulating current, the highest-possible arm current is reduced by 13.4%, and the required circulating current component is analytically derived. The implementation method into the classic control structure of M3C is provided. Performance of the proposed strategy is validated by a simulation benchmark established in MATLAB/Simulink environment in the context of interconnection between two 220 kV/50 Hz power grids.