A Capacitor Voltage Balancing Strategy With Minimized AC Circulating Current for the DC-DC Modular Multilevel Converter

The dc-dc modular multilevel converter (MMC), which has originated from the ac-dc MMC circuit topology, is an attractive converter topology for interconnection of medium-/high-voltage dc grids. Proper operation of the dc-dc MMC necessitates injection of an ac circulating current to maintain its submodule (SM) capacitor voltages balanced. The ac circulating current, however, needs to be minimized for efficiency improvement. In addition, a unique type of imbalance amongst the SM capacitor voltages that is caused by dc power flow needs to be mitigated. This paper proposes a closed-loop control strategy for the dc-dc MMC to simultaneously regulate the dc-link currents, maintain the SM capacitor voltages balanced, and minimize the ac circulating current. Performance and effectiveness of the proposed control strategy are evaluated based on simulation studies in the MATLAB Simulink and experimentally verified on a laboratory prototype.