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.

I. Introduction

The frequency stability of the modern power system is affected significantly by a high penetration of wind power generation. To ensure that the system frequency is within the allowed range, the synchronous generators (SGs) are required to have frequency control capability. For instance, SGs can increase the output power by releasing part of the kinetic energy (KE) stored in the rotor when the system frequency drops owing to generation tripping. As a result, the frequency nadir (FN) is not lower than the predefined threshold and the load-shedding relays are not activated. However, traditional variable-speed wind turbines (VSWTs) lack the frequency regulation capability. The VSWTs have high efficiency because they perform maximum power point tracking (MPPT). However, the MPPT function causes VSWTs to decouple from the grid. When the grid frequency decreases, VSWTs cannot increase the output power to improve the frequency stability. Thus, in some countries, wind farms (WFs) consisting of VSWTs are required to provide the frequency response during a system contingency [1]–[3].