A capacitor-isolated balancing circuit for battery modules applied in grid-tied battery energy storage system

This paper proposes a capacitor-isolated balancing circuit (CIBC) for battery modules in grid-tied battery energy storage system (BESS). The grid-tied converter in the BESS consists of three star-connected H bridges with lower dc bus voltage compared with three-phase converter. To realize required voltage and power, cells are series connected to form battery modules which are connected in series further to achieve high dc bus voltage and large capacity, thus leading to the fact that the voltage of a module is much higher than that of a cell. To realize the module voltage balance, a novel capacitor-isolated balancing circuit topology is proposed. In this balancing circuit the isolation capacitor withstands the offset dc voltage among battery modules instead of semiconductor switches or transformer. The voltage stress on the semiconductor switch is not higher than a battery module voltage. Besides, zero current switching is achieved in the CIBC, which improves the equalization efficiency. In the CIBC transformer is eliminated and CIBC has good modularity. Furthermore, a frequency varying control based on the model of power and impedance is presented, and the control method is easy and effective for the CIBC. The feasibility of the proposed CIBC for battery modules in BESS is validated through simulations.