In a Modular Multilevel Converter-based Battery Energy Storage System (MMC-BESS), the battery current exhibits significant ripples during normal operation. Traditional circulating current injection suppression methods face limitations, including imprecise modeling, reliance on curve fitting, and non-global optimal injection values. To address these issues, this paper proposes a nonlinear optimization-based battery current ripple suppression method for MMC-BESS. By establishing the state-space equations of the MMC-BESS in the synchronous rotating dq reference frame, precise nonlinear analytical expressions for the fundamental and second-harmonic battery current ripples are derived. Subsequently, a nonlinear optimization module for ripple suppression is developed by designing an objective function and corresponding constraints. This module accounts for the influences of both the arm second-harmonic voltage and the second-harmonic circulating current on the peak-to-peak ripple value, enabling real-time calculation of the optimal reference value for the injected second-harmonic circulating current. This approach eliminates the need for arm energy assumptions and bypasses the requirement for curve fitting, thereby significantly simplifying the design process of the ripple suppression controller. Simulation results verify the effectiveness of the proposed method, demonstrating superior suppression performance compared to traditional circulating current injection methods.
Yang et al. (Sat,) studied this question.