Primary frequency regulation strategy for battery-flywheel hybrid energy storage based on adaptive state of charge

With the gradual increase of the proportion of renewable energy in the power system, the risk of frequency fluctuation in the power system increases. Flywheel and lithium battery can complement each other as hybrid energy storage applied in the primary frequency regulation of the grid, effectively solving the problem of system frequency fluctuation. In order to give full play to the respective frequency regulation advantages of flywheel and lithium battery, a primary frequency regulation (PFR) strategy for battery-flywheel hybrid energy storage based on adaptive state of charge (SOC) is proposed. Firstly, a weight allocation PFR is proposed, containing three control methods, namely, positive and negative virtual inertia control and virtual droop control. Secondly, the parameters of the PFR are corrected by using the SOC of the doubly-fed flywheel and the Li-ion battery, so as to improve the PFR capability of hybrid energy storage near the threshold of SOC. Finally, the control strategy of the hybrid energy storage with the other control strategies is compared with that of this paper in simulation under the frequency regulation scenarios. The results show that the SOC fluctuation range of the energy storage system of the strategy proposed in this paper is minimum. The battery will not be overcharged and overdischarged, and the system frequency fluctuation is kept no more than ±0.2 Hz, which proves that the proposed strategy can improve the frequency stability of the grid.