Dynamic State of Charge Balancing Control Strategy for Sodium-ion and Lithium-ion Hybrid Energy Storage System in DC Microgrids

In renewable energy-powered DC microgrids, Battery Energy Storage Systems (BESS) play an indispensable role. Owing to the superior cyclic stability and enhanced safety features of sodium-ion batteries, this paper explores a hybrid energy storage system that integrates both sodium-ion and lithium-ion batteries. This combined approach introduces specific challenges, especially regarding voltage stability and State of Charge (SOC) consistency. To address these challenges, the paper presents a primary controller paired with a nonlinear disturbance observer, ensuring stable bus voltage despite variations in Constant Power Loads (CPLs). Furthermore, a dynamic SOC balancing control strategy is introduced to rectify SOC imbalances stemming from the unique characteristics and capacities of the two battery types. This approach not only maintains consistent SOC levels across storage units, but also meets plug-and-play (PnP) requirements. The effectiveness of the proposed control strategy is validated by simulation results.