Analysis of Capacitor Voltage Unbalance in Hybrid MMC and Its Novel Operation With Reduced Submodule Capacitance

The hybrid modular multilevel converter (MMC) consists of both half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs) in the same arm. The negative insertion of FBSMs allows it to generate higher ac output voltages for a given dc-link voltage than a traditional MMC with only positive insertion. However, the negative voltage in an arm forces HBSMs to bypass as they can only generate positive voltage. The higher the negative voltage, the larger is the duration for which HBSMs are bypassed. This results in an unequal distribution of voltage among the submodules, which further leads to unbalance in their capacitor voltages beyond a certain operating point. This article analyses voltage distribution among the submodules to evaluate the limiting operating point of hybrid MMC beyond which capacitor voltages cannot be balanced. Additionally, a novel operation of hybrid MMC is also proposed, where the arm voltage is generated as a sum of fixed negative voltage and a varying positive voltage, such that the average capacitor voltage increases, resulting in a significant reduction of submodule capacitance required to transfer the same power with the same arm energy as that in the conventional operation. The proposed scheme is verified with detailed simulation as well as experimental results.