Fast Active Balancing System using Multi-level DC Converter for Efficient AC-coupled Battery Systems in Stationary Applications

This paper proposes a cascaded half-bridge multilevel converter (MLC) topology for stationary battery energy storage systems (BESSs). Commonly, many low-voltage battery modules are connected in series to create the DC-link of the inverter. Due to difference in electrochemical characteristics, voltage imbalance happens among different modules during the charge-discharge operation, which negatively affects the lifetime of the battery pack. The proposed cascaded topology allows achieving fast balancing among individual modules. The switching-in and out of each module is done by a half-bridge converter whose output is cascaded with other half bridge converters to form a staircase DC-link voltage. A four-switch H-bridge converter finally alternates this DC bus into multilevel AC voltage. The topology features less harmonics, lower voltage variations, filter-less off grid operation and a reduced size for the interfacing filter inductor with the grid. A hierarchical control strategy is developed to achieve the voltage imbalance mitigation and bidirectional power exchange with the grid. The viability of the proposed converter and effectiveness of the module balancing control are confirmed by numerical simulations and experimental results.