Multi-body Dynamic Characteristics And Bionic Design Of Large-scale Direct Drive Wind Turbine

Nowadays, large-scale offshore wind turbines are becoming more and more popular, andtraditional gear box double-fed wind turbines can no longer meet the need of the fast growingwind turbines market. As a result, direct-drive wind turbines which are famous for their highstability and cost-effective, have become the main direction of wind turbine development.This paper mainly concern on the dynamic characteristic of large-scale direct-drive windturbines. Bionic design based on palm trees is also investigated preliminarily with the help ofexperimental testing and numerical calculation.To build up the multi-body dynamics mathematical model of a direct-drive wind turbine,tower and blades are considered as flexible components, while transmission chain and hub areconsidered as rigid components based on the stiffness distribution features of the wind turbinestructure. Coordinate transformations for flexible and rigid parts are firstly derivedrespectively. Thin-wall beam element theory and successive series of transformations are usedto get the displacement of any point of the blade and tower relative to a global coordinatesystem. Kinetic and potential energy terms of each flexible and rigid body are derived basedon the global deformation. Lagrange equation is finally used to build the system’s governingequation of the mixed flexible-rigid multi-body wind turbine model. A1.5MW direct-drivewind turbine model is designed and its dynamic coupling characteristics are investigated withthe help of finite element method(FEM). Numerical results indicate that there are notdangerous coupling between the main components of wind turbine structure, and thedynamics decoupling design is successful for the example model. Dynamic stiffening is alsotaken into consideration, and the effect of rotation speed on dynamic characteristic of windturbine is obtained. Results show that the effect of dynamic stiffening on high-level naturalfrequency is more obviously than that of the lower-level. Dynamic stiffening is necessary forlarge-scale direct drive wind turbine design.Inspired by the similarity of working environment and structural features between palmtrees and direct-drive wind turbines, experimental study is done to get the stiffness anddensity distribution, and laminated structure characteristics of palm trunks. A bionic tower is designed based on the experimental results. Dynamic buckling and optimal design for lessmaterial use is also undertaken. Numerical results indicate that load capacity of the bionictower increase by126.82%. To better understand the dynamic characteristics of the palmtrees, modal experiment is carried out. Based on the experimental results, a conclusion isderived for reduced the resonance energy of wind turbine structure: when suffering fromlower frequencies resonance, wind turbine should uninstall energy by forcing a single blade tovibrate; when suffering from high frequencies resonance, wind turbine should uninstall energyby introducing a appropriate combined vibration between flexible blades or combinedvibration of blade and tower. However, this conclusion is based on the high flexibility oftower and blade. It is worth mentioning that the use of bionic tower significantly reduces theresonance energy of wind turbine structure.