Small-Signal Behavioral Model and Voltage Control Parameter Optimization for MMC Considering Internal Dynamics

The modular multilevel converter (MMC) has become a pivotal solution for renewable energy integration and HVdc transmission systems credited to its modular scalability, high efficiency, and low distortion. Nevertheless, the intricate internal dynamics, including circulating current and submodule capacitor voltage dynamics, have a significant impact on its control performance. To address this issue, this article proposes a novel equivalent impedance circuit model that incorporates coupled circulating current and capacitor dynamics to optimize ac voltage control for the MMC. Compared with the conventional models, this article: 1) considers the inherent circulating currents and capacitor voltage dynamics, as well as the close-loop voltage control and circulating current control; 2) derivation of a concise two-level converter circuit with series equivalent internal impedance to physically quantify the impact of internal dynamics on output behavior; 3) development of a model-based parameter optimization strategy for ac voltage control that incorporates internal dynamic interference. The proposed model enhances the accuracy of voltage control performance prediction compared to conventional models, enabling targeted controller tuning that improves voltage dynamic response and stability performance. Finally, detailed simulation and experiment results under various conditions verify the accuracy of the proposed model and the effectiveness of the control optimization strategy.