A modular multilevel converter with active power filter (APF-MMC) under low-frequency operation

In the realm of medium/high voltage applications, the modular multilevel converter with an active power filter (APF-MMC) emerges as a technology that eliminated the inherent voltage fluctuations of larger sub-module (SM) capacitors. However, the introduced APF circuit in each phase can only deal with power in even frequencies, and the APF-MMC cannot be directly applied in the field of motor drives with low-frequency operation. In this paper, based on the APF-MMC topology, by adding two high-frequency variables as control degrees of freedom, the base frequency power is transferred to high-frequency power, which can considerably minimize the capacitor voltage ripple in the low-frequency region of the topology. In addition, the influence of the injected high-frequency variables on the output characteristics of the topology is eliminated by controlling the APF circuit as hardware degree of freedom. Its equivalent circuit and operating principle are introduced in detail. Concurrently, a control method is proposed for the APF circuit, ensuring seamless operation of the converter. Finally, through a combination of simulation and experimental results, it is demonstrated that the APF-MMC topology surpasses the conventional MMC in its efficacy under low-frequency operation.