Integrating a battery energy storage system (BESS) with a modular multilevel converter (MMC) enables the HVDC converter to deliver active power and provide frequency support during transient events. However

delivering frequency support may cause a decrease in the MMC capacitor voltage

and parameter inconsistencies among the storage units can further lead to state-of-charge (SoC) imbalance

posing risks to the system’s safe and stable operation. To address these challenges

this paper proposes a distributed MMC-BESS frequency support control method that accounts for SoC balancing. First

a versatile static synchronous machine control strategy is adopted to enable autonomous frequency response. Second

a secondary power allocation mechanism is introduced

allowing the battery to rapidly provide active power support. Finally

an SoC balancing strategy is designed to dynamically adjust the power references of battery

achieving internal energy balancing while maintaining effective frequency support. Results based on PSCAD/EMTDC and HIL experiments verify that the proposed method not only provides active frequency support but also improves SoC consistency among storage units

thereby enhancing system stability and sustained operation capability.