Bank Division Topology for Existing Hierarchical Balancing Control in a Grid-Scale Battery Energy Storage System

Abstract

Lithium-ion (Li-ion) cells are becoming increasingly attractive for use in grid-scale Battery Energy Storage Systems (BESSs). A key problem with BESSs is the potential for poor utilization of mismatched cells and reliability issues resulting from the use of large series strings of cells. This paper investigates the close integration of a cascaded H-bridge multi-level inverter and a large number of Li-ion cells interfacing with an AC electrical grid. The cells are organized in a hierarchical structure consisting of modules, sub-banks, banks and phases. The control strategy includes four levels of balancing: balancing of cells within a module, balancing of modules within a sub-bank, sub-banks within banks, and banks within phases. The system is validated in simulation for a 380 kWh BESS using 2835 Li-ion cells. Charge balancing is demonstrated for mismatched cells by varying the parameters such as ampere-hour capacity, internal resistance, and initial State-of-Charge. Bank division topology has been implemented to look for the required cells to achieve the reference voltage by keep dividing the cells into two banks. It reduces the simulation time by 73.5% and thus improves the operational efficiency of the balancing control while achieving cell balancing. This work is intended to address the challenges of eventual scaling towards a 100 MWh+ BESS, which may be composed upwards of 100k individual cells.