Reactive-Power-Assisted Frequency Recovery Control for High-Speed Flywheel Virtual Synchronous Generators under Frequency Drop Conditions

To address the issues that, during frequency dips, grid-forming virtual synchronous generators (VSGs) relying solely on active-power inertial support may readily cause a decrease in the point of common coupling voltage, an increase in the power angle, and a relatively slow grid frequency recovery, this paper proposes a reactive power control method for the inertial response of high-speed flywheels. Starting from the relationship between the internal equivalent electromotive force of the VSG and reactive power regulation, the proposed method constructs a reactive power reference based on the internal voltage demand, and designs compensation terms by incorporating the rate of change of frequency and the power-angle deviation, enabling reactive power support to be enhanced promptly during frequency drops. The strategy takes both steady-state voltage regulation and fast transient response into account, and can increase the voltage at the point of common coupling during frequency dips, thereby reducing grid disturbances, enhancing active-power support capability, and accelerating system frequency recovery. Compared with conventional reactive power control methods based only on voltage deviation, the proposed method better reflects the coupling relationship associated with VSG inertial support, providing a concise and explicit control approach for frequency-voltage coordinated control of high-speed flywheel grid-forming inverters in power grids.