An Adaptive Additional Control Strategy for Suppressing Low-Frequency Grid Oscillations in Doubly-Fed Wind Farms

Abstract

Aiming at the problem of weakly or negatively damped low-frequency oscillations caused by cross-zone transmission of electricity from large wind farms, this paper proposes a fast terminal sliding-mode additional damping controller based on the Lyapunov stability theory. By investigating the flexible power regulation characteristics and the capability of dynamic frequency response to damping regulation of doubly-fed wind turbines (DFIG), a rotor magnetic chain controller is designed according to the relationship between the applied voltage and magnetic chain of DFIG rotor and the sliding mode variable structure control method. When low-frequency oscillations occur in the system, the desired magnetic chain value will deviate from the actual magnetic chain value. The additional damping controller outputs an adaptive control signal for the rotor-side power control link to increase the active output of the wind farm and suppress low-frequency oscillations in the system. A simulation model of the wind power grid-connected system is established in MATLAB/Simulink for off-line simulations, and a real-time simulation experiment of a large wind farm cross-zone transmission model based on real time digital simulation system is conducted. The results of both off-line and real-time simulations show that when low-frequency oscillations occur in the system, the proposed control method can quickly regulate the active power emitted by the DFIG and enhance the damping level of the system, which is effective in suppressing low-frequency oscillations in the system.

Keywords

• doubly-fed wind turbines (dfig)
• frequency stability
• low-frequency oscillation
• lyapunov stability
• sliding mode control