State-of-Charge Balancing Control for ON/OFF-Line Internal Cells Using Hybrid Modular Multi-Level Converter and Parallel Modular Dual L-Bridge in a Grid-Scale Battery Energy Storage System

Cell state-of-charge (SoC) balancing within a battery energy-storage system (BESS) is the key to optimizing capacity utilization of a BESS. Many cell SoC balancing strategies have been proposed; however, control complexity and slow SoC convergence remain as key issues. This paper presents two strategies to achieve SoC balancing among cells: main balancing strategy (MBS) using a cascaded hybrid modular multi-level converter (CHMMC) and a supplementary balancing strategy (SBS) using a cascaded parallel modular dual L-bridge (CPMDLB). The control and monitoring of individual cells with a reduction in the component count and the losses of BESS are achieved by integrating each individual cell into an L-bridge instead of an H-bridge. The simulation results demonstrate a satisfactory performance of the proposed SoC balancing strategy. In this result, SoC balancing convergence point for the cells/modules is achieved at 1000 min when cell-prioritized MBS-CHMMC works without SBS-CPMDLB and at 216.7 min when CPMBS-CHMMC works together with SBS-CPMDLB and when the duration required reduces by 78.33 %. Similarly, a substantial improvement in SoC balancing convergence point for the cells/modules is achieved when module-prioritized MBS-CHMMC works together with SBS-CPMDLB; the duration needed to reach the SoC balancing convergence point for the cells/modules is achieved after 333.3 and 183.3 min.