A Combined M3C Suitable for Motor Drive Systems Operating at Close or Equal Input–Output Frequencies

Modular multilevel matrix converter (M3C) is a topology that can directly realize the AC-AC power conversion and is widely used in high-power motor drive applications. However, when the motor frequency is close to the grid frequency, the voltage fluctuation problem of the sub-module capacitors worsens. The traditional method of reducing capacitor voltage fluctuations is to inject common mode voltage and circulating current, resulting in the increase of voltage and current stress of the bridge arms. In this paper, a new topology of M3C combined with cascaded H-bridge is proposed, which can effectively reduce fluctuations in the capacitor voltage of the M3C bridge arm submodule when the input and output frequencies are close or equal, while it still has the advantages of the traditional M3C such as easy modular design, low output harmonics, and strong scalability. Firstly, a mathematical model of the combined M3C topology is established. With the model, the cause of the capacitor voltage fluctuations of the M3C bridge arm is analyzed, and its mathematical expression is derived when the motor frequency is close or equal to the grid frequency. Then, a capacitive compensation method using cascaded H-bridges is proposed to provide the reactive power required by the motor to reduce fluctuations in the capacitor voltage of the M3C bridge arm submodule under the close or equal frequency working condition. Compared with the conventional methods, the combined M3C achieves smaller voltage fluctuations of the capacitors, eliminates the need for common-mode voltage and circulating current injection, and achieves lower power loss. Finally, the correctness and effectiveness of the proposed topology and control strategy are verified by simulations and experiments.