Four-quadrant Hybrid MMC and its Circulating Current Decoupling Control Strategy

This paper proposes a hybrid modular multilevel converter(H-MMC) topology, which is suitable for high-voltage and high-power four-quadrant variable-speed drive applications. In this topology, two anti-parallel thyristors are inserted into the DC bus of back-to-back MMC, which can realize bidirectional power transfer and effectively solve the problem of excessive capacitor voltage fluctuation of motor-side MMC at low speed. In addition, by establishing the equivalent circuit and mathematical model of the DC circuit, this paper reveals the conflict of DC current control between the motor-side and grid-side MMC. To solve this problem, a circulating current decoupling control of grid-side MMC is proposed to handle this issue, which includes DC, fundamental frequency and even-order harmonic components control loop design. Moreover, by taking advantage of the discrete nature of the digital controller, it is possible to ensure that the output common-mode component of the three-phase cyclic controller is zero, so that the effect on the DC current can be avoided. Finally, the effectiveness of the proposed topology and decoupling control strategy during full frequency range is validated by the simulation and experiment results.