Coordination control strategy for battery-ultracapacitor hybrid energy storage system in microgrids with unbalanced and nonlinear loads

Hybrid energy storage system (HESS) is widely used in microgrids, and its research mainly focuses on energy management, power allocation, topology design and so on. For the power allocation issue, most research only involves the fundamental power allocation within a single HESS. While the allocation of reactive power, negative sequence power and harmonic power, and the coordinated operation of multiple HESS units are rarely be studied. In response to the above situation, this paper proposes a coordination control strategy for the HESS group under the unbalanced load and nonlinear load conditions. The proposed strategy is based on droop control method and can realize interconnection and power sharing for multiple HESS units under weak communication condition. Inside the HESS, the battery (BAT) power conversion system (PCS) works in droop mode, providing energy and only outputting fundamental active power; and the ultracapacitor (UC) PCS works in the compensation mode, analyzing the output power of HESS and providing the reactive, negative sequence and harmonic powers. This strategy can provide better system performance in unbalanced and nonlinear load conditions. It utilizes UC-PCS to deal with the inherent power sharing issues of droop control method, and enhance the transient process. Meanwhile, the stability of microgrid and the service life of the BAT-PCS are increased with the improvement of power quality. Moreover, the HESS adopts a dual inverter structure, which is conducive for the upgrading of existing equipment. Finally, the validity of the proposed strategy is verified by simulation and experimental results.