研究目的
To propose an improved capacitor voltage balancing method for multimode operation of modular multilevel converter with integrated battery energy storage system (MMC-BESS) that addresses the state-of-charge (SOC) inconsistency among batteries and enhances the SOC equalisation rate.
研究成果
The proposed capacitor voltage balancing control method is effective for all operation modes of MMC-BESS, significantly improving the SOC equalisation rate. The optimisation of ac and dc modulation indexes adjustment enhances the tolerance of unbalanced power among SMs within phase arm. The small-signal models provide an analytic basis for the capacitor voltage balancing controller design, validated by simulations and experimental results.
研究不足
The study acknowledges the technical constraints related to the SOC inconsistency among batteries and the need for optimising the combination of ac and dc modulation indexes to enhance the tolerance of unbalanced power among SMs within phase arm. It also mentions the potential for over-modulation at the MMC side if the SOC equalisation coefficient is not properly selected.
1：Experimental Design and Method Selection
The study employs an improved capacitor voltage balancing method by adjusting ac and dc modulation indexes simultaneously. The methodology includes the use of carrier phase-shifted carrier PWM (CPS-PWM) for fixed switching frequency and even power loss distribution.
2：Sample Selection and Data Sources
The study uses a three-phase MMC-BESS model built in MATLAB/Simulink for simulations and a downscaled prototype for experimental validation.
3：List of Experimental Equipment and Materials
The main parameters include grid voltage, dc-link voltage, rated apparent output power, ac line frequency, arm inductance, SM number per arm, SM capacitance, rated battery voltage, battery side dc/dc converter inductance, rated battery capacity, switching frequency at battery side, and carrier frequency at MMC side.
4：Experimental Procedures and Operational Workflow
The experimental procedure involves comparing the proposed capacitor voltage balancing method with conventional methods under different operation modes, verifying the SOC equalisation rate enhancement, and validating the control strategy through simulations and experimental results.
5：Data Analysis Methods
The study analyzes the tolerance of unbalanced power among SMs within phase arm, optimizes the combination of ac and dc modulation indexes, and conducts small-signal models for closed-loop controls of ac and dc modulation indexes.
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