High-Frequency Injection Parameter Optimization Design Strategy for Modular Multilevel Converters Based on Steady-State Boundary Evaluation

To address the challenges of traditional high-frequency injection methods in suppressing low-frequency ripple in the capacitor voltage of Modular Multilevel Converters (MMCs), which depend heavily on empirical formulas for parameter design, this paper proposes a parameter optimization framework based on steady-state boundary evaluation to broaden the full-frequency operation capability. First, through analyzing the interplay between system constraints and control degrees of freedom, a hierarchical optimization strategy is developed to accurately define the feasible solution space in a multi-parameter environment subject to multiple constraints. Based on this analysis and aiming to extend the operational boundary, a quantitative design approach for the injection frequency is presented, which effectively eliminates theoretical limitations associated with traditional empirical formula-based designs. Additionally, by considering the characteristics of the constraint space and focusing on minimizing injected circulating current, adaptive regulation of key injection parameters is achieved under all operating conditions. Experimental results indicate that, compared with existing methods, the proposed parameter optimization approach not only alleviates over-control and under-control issues throughout the entire frequency range but also significantly reduces arm current stress while complying with steady-state constraints, thereby improving the operational capability of MMC.