A Coordinated Control Strategy of Flywheel-Battery Hybrid Energy Storage System for Participating in Grid Frequency Regulation

High penetration of renewable energy in the power grid brings many technical challenges to grid security operation and stability control such as grid frequency regulation, due to the intermittence and fluctuation of renewable energy sources. Flywheel-battery hybrid energy storage system (HESS), which has the advantage of combining power-type energy storage with energy-type energy storage, has the characteristic of high-energy density and high-power density, and is therefore capable of providing the grid frequency regulation support. Based on the model of flywheel-battery HESS, a coordinated control strategy for grid frequency regulation is proposed in this paper. A group of proportional coefficients, which are modified in real time according to the deviation and the rate of change of frequency, are used to adjust the output active power of each energy storage system in different stages of dynamic frequency regulation. The virtual inertia method and the virtual droop method, whose control coefficients are modified based on the state of charge, are used to control flywheel energy storage (FESS) and battery energy storage (BESS), respectively. The proposed control strategy can comprehensively utilize the advantage of two types of energy storage systems, realize BESS and FESS coordinatively participating in frequency regulation and improve the frequency regulation performance of HESS. The simulation results based on the PSCAD software verify the feasibility and effectiveness of the proposed control strategy for grid frequency regulation.