A DFIG Based Stepwise Inertia Control Method for Fast Frequency Response Considering Secondary Frequency Drop

Doubly-fed induction generators (DFIGs) can participate in power grid frequency regulation utilizing stepwise inertia control (SIC) by releasing the kinetic energy stored in the rotational masses. Nevertheless, a secondary frequency drop (SFD) of the power grid may occur during the rotor speed recovery stage of conventional SIC in DFIGs, which will further worsen the system unbalance and cause a second frequency nadir. To address this issue, this paper proposes an improved SIC considering SFD. In particular, the stored kinetic energy of DFIGs is quantified under various wind speeds. Further, the proposed SIC is divided into the overproduction stage and rotor speed recovery stage, with appropriately designed shaping parameters for each stage to strike a balance between the SFD prevention and the recovery speed. The performance of the proposed SIC is compared with conventional SIC and frequency-based inertia control (FBIC) through explicit simulation analysis. The results verify that the proposed control strategy can effectively offset the SFD while improving the frequency nadir and the rate of change of frequency (RoCoF) in any wind conditions, as well as contribute to reliable operation during an unexpected second cascade event.