Virtual-Impedance Pre-Synchronization of Flywheel Energy Storage Active-Support Motor Equipment Based on Soft-Start Control Strategy

Active-support motor equipment combining a doubly fed induction machine (DFIM) with a flywheel energy storage unit possesses fast active and reactive power support capability, exhibiting promising prospects for enhancing power system stability. However, the startup process of a DFIM involves multiple dynamic stages, including flux linkage establishment, excitation converter connection, and rotor-speed acceleration, which may result in large starting currents, electromagnetic-torque fluctuations, and converter power transients. And existing soft-start methods still involve trade-offs among startup inrush suppression, additional hardware requirements, control complexity, and startup performance. Moreover, the coordinated control of rotor-side closing synchronization, closed-loop acceleration, and converter power constraints during the startup of active-support motor equipment requires further investigation. To address these issues, this paper proposes a rotor-side pre-synchronization method based on virtual impedance. A virtual impedance model is established between the rotor-induced electromotive force and the output voltage of the excitation converter. According to the voltage difference and the virtual impedance, a virtual current is calculated, and rotor-side pre-synchronization is achieved by regulating this virtual current to zero, thereby effectively suppressing the inrush current at the closing instant. On this foundation, a complete soft-start control strategy is formulated. Finally, simulation and hardware-in-the-loop (HIL) experiments are carried out to verify the method. The results demonstrate that the proposed strategy reduces the absolute peak value of the three-phase rotor current from 157 A to 21 A, the rotor-current RMS value during the first electrical cycle after closing from 91.287 A to 9.614 A, and the DC-link voltage overshoot from 2.81% to 0.078%, corresponding to reductions of 86.62%, 89.47%, and 97.22%, respectively. These results confirm that the proposed strategy can effectively suppress the rotor-side closing inrush current and improve the smoothness and reliability of the startup process of the active-support equipment.