State-of-Charge Balancing Control for Optimal Cell Utilisation of a Grid-Scale Three-Phase Battery Energy Storage System Using Hybrid Modular Multilevel Converter Topology Without Redundant Cells

Cell state-of-charge (SoC) balancing within each branch of a three-phase battery energy storage system (BESS) and among three branches is crucial to overcome the inability to fully utilise the available capacity of a three-phase BESS. The proposed topology is constructed with one branch instead of three branches to take advantage of its idle cells/modules (Ms) (one-third of the total cells/ Ms) and to eliminate the need of SoC balancing among the branches. Contrary to conventional topologies, idle cells/ Ms can serve as redundant cells/ Ms or can be dropped out of a BESS, thereby leading to a reduction in the cost, control complexity, size, and losses of a BESS. A novel SoC balancing strategy for the proposed topology of a three-phase BESS is introduced in this paper. Moreover, the cell/M activation algorithm is implemented to minimise the duration needed to activate the cells/Ms required to generate voltage for the phases, thereby leading to an improvement of battery operational efficiency. Based on the simulation results, SoC balancing among 3996 cells, 2664 cells, 333/222 Ms, and 12 cells in M with the lowest and the highest average SoC is achieved in 53 min, 48 min, 38 min, 18 min and 53 min, respectively.