Approximation to Frequency Control Capability of a DFIG-Based Wind Farm Using a Simple Linear Gain Droop Control

In the literature, many droop control loops have been proposed to enhance the frequency control capability of the wind farm (WF) when a large frequency disturbance occurs. However, some loops are not feasible to be implemented in the dynamic equivalent model (DEM) of the WF. The kinetic energy (KE)-based droop control loop is an example. The droop gain of this loop is expressed as a quadratic function of the wind turbine (WT) rotor speed. However, it is not feasible to implement a nonlinear function in DEM of WF. Therefore, in this study, a new linear-gain droop control loop is proposed for the doubly fed induction generator-based WF. In the proposed control loop, the droop gain is a linear function of the WT rotor speeds. By selecting the proper coefficients of the linear function, the proposed linear droop gain can achieve a good approximation to the quadratic droop gain. The performance of the proposed droop control loop is demonstrated based on three initial conditions. To verify the responses of system frequency and WF power output, four indices are developed. The simulation results demonstrate that the proposed linear-gain droop control loop is capable of approximating closely to the KE-based droop control loop.