Modified Modular Multilevel Converter

The modular multilevel converter (MMC) is receiving extensive research interests in high/medium voltage applications due to its modularity, scalability, reliability, high voltage capability and excellent harmonic performance. The submodule capacitors are usually quite bulky since they have to withstand fundamental frequency voltage fluctuations. To reduce the capacitance of these capacitors, this paper proposes a modified MMC with active power decoupling circuit within each submodule. The modified submodule contains an auxiliary half bridge, and the submodule capacitor is split in two. The midpoints of the half bridge and the split capacitors are connected by an inductor. With the modified submodule, the fundamental frequency voltage fluctuation can be suppressed drastically. The second-order voltage fluctuation, which is also the second most significant component in the submodule voltage fluctuations, is removed by proper control of second-order circulating current. As a result, the submodule capacitance is reduced significantly. The viability and effectiveness of the proposed new MMC are confirmed by the simulation and experimental results. The proposed MMC is best suited for MV applications where power density is given a high priority.