Modular Multilevel Converter With Three-Port Power Channels for Medium-Voltage Drives

Due to advantages such as modular design, scalability, and excellent harmonic performance, modular multi-level converter (MMC) has drawn more and more attention in high-power and high/medium-voltage application areas, including medium-voltage (MV) drives. However, the submodule (SM) capacitors must withstand fundamental-frequency ripple power, the amplitude of which is proportional to the output current. The huge low-frequency fluctuation of the SM voltage at startup then becomes a major obstacle in MV-drive applications. The mainstream, common-mode injection methods are not without problems. This paper proposes a modified MMC topology to eliminate the voltage fluctuation. Three-port power channels are added, each to laterally connect three SMs that belong to the three phases, respectively. Since the sum of the three-phase pulsating powers is zero, the power channels provide a means for the powers to cancel out with each other. A cost-benefit analysis is given in comparison with traditional methods. Simulation and experimental results proved the validity of the proposed topology.