SOC Balance Control Method for Cascaded Energy Storage System Based on Nearest Level Modulation

To address the issue of the in-phase state of charge(SOC) unbalancing in a cascaded H-bridge battery energy storage system, this paper proposes a novel control strategy based on nearest level modulation(NLM). Through NLM, this paper first calculates the number of sub-modules needed, then adjusts the priority of sub-modules in different SOC by the charging state, so that the sub-modules with larger SOC deviation converge to the average value. Considering that sub-modules with a large SOC deviation participating in charging or discharging may bring distortion of the PCS output, limiting the number of sub-modules with large deviations during the charging or discharging is needed, so that the sub-modules with large SOC deviation and sub-modules close to average SOC cooperate in charging/discharging process to reduce the distortion of PCS output. The simulation result verified the correctness of the strategy.