Development of open-loop and closed-loop techniques for the optimization of modular multilevel conversion structures

New control and modulation techniques are proposed for the reduction of the DC-link capacitors in cascade multilevel converter structures for medium-voltage AC motor drives. Two different approaches have been followed, the first one based on the synthesis of suitable modulating signals, and the second one based on the feedback of the motor currents and of the DC-link voltage in each H-bridge module. A new DC-link passive filtering approach is also proposed, which has been studied and optimized with reference to such techniques. The explanation of the proposed techniques and the most significant analyses and results are reported