An Optimal Fast Frequency Control Method for Variable Speed Wind Turbines Based on Doubly Fed Induction Generators Through Simultaneous Control of Frequency and Maximum Torque

Abstract

The need to mitigate environmental pollution and conserve the environment has led to the rapid expansion of renewable energy sources (RESs) and wind farms (WFs) in power systems. Thus, the involvement of WFs in frequency control and raising the frequency nadir (FN) under transient conditions of the system will be highly significant and indispensable. This research presents a proposal to enhance the system frequency by utilizing WFs and restoring the speed of the wind turbine (WT) rotor using the doubly fed induction generator (DFIG) while avoiding frequency second dip (FSD). In this design, once the disturbance and decrease in the system frequency are identified, the new power reference is incorporated into the maximum power point tracking (MPPT) characteristic as a function of two parameters, the changes in system frequency and the speed of the WT rotor, taking into account the torque limit. During frequency support, the frequency change parameter increases, while the rotor speed parameter decreases. This results in a smaller decline rate of the reference power, so that the electrical power goes below the mechanical power with a smooth gradient, not a step-wise manner. As a result, the rotor speed is restored fast without FSD. Another benefit is that the MPPT characteristic qualities are maintained during the frequency support, as the new reference value is incorporated into the MPPT characteristic. The MATLAB simulation results of the test system demonstrate that the proposed design successfully enhances the system’s frequency without inducing a FSD. Additionally, it effectively and rapidly restores the rotor speed.