Small Signal Modelling Of Doubly-fed Induction Generator Wind Turbines At The Electromechanical Time Scale

Large-scale wind power integration will impose great impacts on the power system small signal stability(SSS), and the weak grid connection together with the long distance transmission of the wind power will further introduce new threats to wind turbine(WT) electromechanical dynamics and power system damping. Small signal modelling of the doubly-fed induction generator(DFIG) WT at the electromechanical time scale(0.1~2.5Hz) is the basis for the SSS analysis of power systems with doubly-fed wind power generations. This paper proposes a novel reduced order model of the DFIG-based WT for SSS analysis, and the internal voltage of DFIG is defined. The well validated model is applied for SSS studies and several conclusions are made.The fundamental principles of the DFIG-based WT are introduced. Based on the SSS research purposes, the simplification method for dynamics at different time scales is discussed. And the small signal model of the DFIG-based WT is established. The presented model ignores the stator/rotor transients, the inner current-control loops and the DC-link dynamics, and highlights the necessity of the phase-locked loop(PLL) modelling. The internal voltage of the DFIG is defined, with which we can have an insight into the excitation control and the synchronization control. Modal analysis and time domain simulations verify the model well.The proposed model applies for DFIG-based WT SSS researches and power system low frequency oscillation studies. This paper analyzes the impacts of different working points, PLL parameters, grid strength and drive train parameters on the wind generator small signal dynamics, and reveals the changing laws of the wind generator modes as the working points change. The PLL is found to have a significant influence on the WT generator outputs, especially in weak grid conditions when the outputs, the reactive/voltage control loop and the PLL are coupled. The impacts of the working points and the PLL on power system intra/inter-area damping is analyzed, and further illustrated via the internal voltage presented.