Coordination control of lithium battery-supercapacitor hybrid energy storage system in a microgrid under unbalanced load condition

For a microgrid, it is important to supply high quality powers to consumers. However, due to the unbalanced load condition, the power quality of a microgrid is always weak. Normally, by the control method of negative-sequence voltage, the microgrid voltage can be enhanced. However, the unbalanced current problem is still unsolved for those main converters. In this paper, according to the hybrid energy storage devices, a coordination control strategy is proposed to enhance the power quality for microgrid under unbalanced load condition. The hybrid energy storage system (HESS) is composed of a lithium battery power conversion system (li-pcs) and an ultra-capacitor power conversion system (uc-pcs). The HESS employs li-pcs as the main unit to regulate power and the sc-pcs as the auxiliary unit to absorb unbalanced power. In this way, the main voltage source device of the microgrid, li-pcs, keeps a reasonable operation all the time. In addition, the HESS can operate in three modes: PQ mode, VF mode and Droop mode. While using Droop mode, several HESSs can operate together just like the feature of the distributed generation (DG) units with droop control method. Finally, Matlab simulation and experiment results verify the validity of the proposed strategy.