Small-Signal Stability Analysis of DFIG-based Wind Turbines Equipped with Auxiliary Control Systems under Variable Wind Speed

Providing ancillary services such as frequency support and power operating reserve by doubly fed induction generator-based wind turbines has received considerable attention in the recent years, and several auxiliary controllers have been proposed to provide these services. In this paper, small-signal stability of wind turbines equipped with these controllers is assessed in presence of a frequency event and varying wind speed conditions. For this purpose, first, several auxiliary controllers are discussed, and a unified auxiliary controller (UAC) is proposed, which can represent all of these controllers simultaneously. Then, the overall transfer function from the wind speed to the wind turbine rotor speed considering the proposed UAC is derived. Moreover, by neglecting the power grid effects, an approximated first-order transfer function is also obtained. Exhaustive simulations examine the system stability with UAC, under both computer-generated and real wind speed profiles. In addition, simulation results confirm validity of the obtained detailed transfer function and the accuracy of its approximated version for several auxiliary controllers, by selecting appropriate UAC parameters.