Current Controlled Operation of Cascaded H-Bridge Converter for Fast SoC Balancing in Grid Energy Storage

In grid energy storage systems based on batteries, the interface converter topology plays a significant role in the optimized operation of the system. Among different types of multi-level converters, cascaded H-bridge converter (CHBC) provides the advantage of utilizing isolated battery pack (BP) with a lower number of series-connected cells and can perform state of charge (SoC) balancing by controlling the power flow to/from each BP. However, the rate of SoC balancing is limited due to low balancing current when the amount of power exchange is low. This paper proposes a new power control method using hybrid modulation strategy with extended operating region (HMSEOR) which enables the CHBC to perform fast SoC balancing at constant current irrespective of the amount of power exchange between the battery system and the grid. The HMSEOR allows power flow from a BP with a higher SoC to a BP with a lower SoC without compromising power exchange with the grid. The performance of the proposed SoC balancing method is validated through real-time hardware-in-the-loop (HIL) simulation and compared with the conventional SoC balancing at unity power factor operation.