An Enhanced Closed-Loop Control Strategy With Capacitor Voltage Elevation for the DC-DC Modular Multilevel Converter

The dc-dc modular multilevel converter (MMC), derived from the ac-dc MMC, is an attractive converter topology for interconnection of medium-/high-voltage dc grids. The dc-dc MMC generates an ac voltage component in each arm to drive an ac circulating current for maintaining the energy balance of its submodule capacitors. The power transfer capability of the dc-dc MMC and the amplitude of the ac circulating current are closely coupled with the available arm ac voltage headroom. As the voltage conversion ratio of the dc-dc MMC deviates from 0.5, the power transfer capability of the converter diminishes. This paper proposes an enhanced closed-loop control strategy, which increases the power transfer capability and at the same time reduces the ac circulating current of the dc-dc MMC. Performance and effectiveness of the proposed control strategy are demonstrated by simulation studies in the MATLAB/Simulink as well as experiments conducted on a laboratory prototype.