Abstract Summary
Stability analysis for a large-scale wind turbine has been an increasing topic in recent years, but its intrinsic mechanism is still blurring. This paper aims at developing an explicit and fully linearized Aero-Hydro-Struc model to reveal the instability phenomena and giving a novel insight into understanding the instability mechanism. This paper introduced a series of evaluation procedures and methods for the stability judgment of a floating offshore wind turbine (FOWT) based on a linearized model. The linearized model contains linear aerodynamics model and hydrodynamics model. The nonlinear aerodynamics model including the widely-used steady engineering model - Blade Element Momentum theory (BEM) - for an operating wind turbine, and a simpler model for parked wind turbines is also introduced, they are both linearized in this paper. Besides, Morison’s equation and potential flow theory are adopted for the hydrodynamics model and their linear versions are introduced in this paper. Linear versions of aerodynamic load models are developed by means of the first-order Taylor’s expansion without considering the second-order items, and the stability analyses are conducted based on the linear models using the state-space method, as long as the implicit and nonlinear aerodynamic and hydrodynamic loads are successfully linearized as static forces and damping matrices. Stand-still, operating and parked (with yaw angles) wind turbines have been analyzed through the proposed method in this paper. It is found that the platform is unlikely to suffer from instability under these operational conditions as the hydrodynamic damping participates prominently, while the blade edgewise and the tower Side-Side (SS) movements are more possible to experience instability for a parked wind turbine with yaw misalignments, since the aerodynamic damping is negative and the hydrodynamic damping is not taken into account in these degree of freedoms (DOFs), and these two kinds of instabilities are caused by stall-induced vibrations. It’s also observed that the instabilities of blade and tower DOFs mainly arise from the blade root airfoil sections since the angles of attack are particularly high because of the higher twist. Besides, blade flapwise and edgewise, as well as the tower Fore-After (FA) and SS movements, are proved always highly damped for a normally operated wind turbine, which indicates the possibilities of the instability for these DOFs remain low. Moreover, the time-series and frequency-domain responses for an operating wind turbine have been studied as well, it’s found the blade vibrations in frequency-domain are dominated by the rotor rotation speed and are coupled with the platform and wave frequencies, while the tower responses are related to the rotor rotation speed, platform frequencies, wave frequencies, and the tower natural frequencies.
