Capacitor Voltage Feedforward-Based Voltage Loop Control for Grid-Forming Modular Multilevel Converters Under Wide Range of Grid Strength

An efficient closed-loop voltage tracking control (VTC) for a grid-forming-based converter is required for precise voltage regulation. However, few papers designed a VTC exclusively for modular multilevel converter (MMC) with a full understanding of the MMC's internal dynamic coupling and presented a detailed analysis of the controller. In contrast, traditional voltage-current double loop control or its derivatives are employed without considering the impact of the coupled fluctuating capacitor voltage and with insufficient stability analysis under various operating conditions, especially for the wide range of grid short-circuit ratio. To solve these problems on MMC, this article systematically analyzes the effect of each component of capacitor voltages on the output voltage. In this article, an easy-implementalVTCis proposed which takes the dominant factor of capacitor voltage into the feed forward path (CVF). The proposed CVF-VTC with an appropriate feed forward coefficient shows a high damping effect under varying grid strength with the eigenvalue proof of the established small signal model. Moreover, the PI parameters of the VTC show a low effect on system stability when CVF is applied. Experiment results are performed to verify the proposed method.