Research On Multi-body Dynamics Model Of Wind Turbine And Its Application

In order to capture more energy from the wind,wind turbines are developing in a large-scale direction with higher power,longer blades and higher towers,and the structure of the wind turbines has become more complex than before.At the same time,in order to reduce the weight of blades,save the cost,and for different design requirements,the blades are often designed as a slender form with outstanding flexibility and complex shapes such as pre-bend,swept and twist.All these have led to more serious dynamic problems of wind turbines such as geometric nonlinearity.aeroelasticity,resonance,and instability,so it is necessary to establish new dynamics analysis tools of wind turbine to carry out loads and operational safety assessment.Therefore,this paper establishes a new dynamic response analysis model and simulation analysis platform which is suitable for modern large-scale wind turbines and its blades.Based on theoretical analysis,numerical calculation and experimental simulation,the dynamic problems such as geometric nonlinearity of blades,modal vibration of free vibration and rotational vibration,and wind rotor imbalance are investigated.Firstly,a fast analysis model named BaMB(Blade analysis with Multi-Body)of wind turbine blades was established based on the absolute coordinate method in the theory of multi-body dynamics.The model was successfully converted into MATLAB code and its reliability was verified.In the BaMB model,the blades are discretized into a multi-rigid body system composed of spherical hinges,springs,and dampers.The deformation of the blade is equivalently replaced by the rotation of the spherical hinge in space.The elastic force generated by the blade resisting deformation is equivalently replaced by the equivalent elastic force of spring obtained by the three-dimensional multi-rigid body discrete element model.The BaMB model can accurately predict the deformation of blades with less degrees of freedom and has geometric nonlinear analysis capabilities.In addition,the model can describe the complex geometric shape of blades such as pre-bend,torsion angle,and swept.Moreover,the dynamic equation of the BaMB model already contains the inertial force generated by rotation,which can be used to analyze the dynamic characteristics of wind turbine blades in the rotating state.What's more,the BaMB model discretizes the flexible blade into a multi-rigid body system,which is suitable for any multi-rigid system or rigid-flexible coupling multi-body system.Therefore,the model can be extended to the wind turbine after changing the constraint equation and the equivalent elastic force of the spring and giving a reasonable initial coordinate value and external load.The wind turbine aero-structure-control coupling model named ARC(Aero-structure-control coupling)was established by combining the structural dynamics model based on the BaMB model,the BEM aerodynamic model,the variable-speed and variable-pitch control model,the wind model including shear wind and turbulent wind.The detailed expressions of model aerodynamics,structure,control and load calculation methods are given in this paper,and the model is converted into MATLAB code.In the ARC model,the wind turbine is simplified as a multi-body system consisting of blades,hub,nacelle and tower.The hub and the nacelle are both modeled as a single rigid body,and the blades and the tower are discrete into multi-rigid bodies.Additionally,the ARC model takes into account the effects of aeroelastic and the coupled vibration of the wind rotor and tower.Furthermore,the ARC model can realize the dynamic response analysis of the wind turbine under steady wind,shear wind and turbulent wind.What's more,the three blades of the ARC model are independently modeled,which lays the foundation for the investigation of the rotor imbalance.The research object of geometrical nonlinearity is a straight 100 kW blade and a large pre-bend 2.3 MW blade.The simulation results of the traditional Euler-Bernoulli beam model,Timoshenko beam model,finite element model and BaMB model under static loading conditions are compared with experimental values.The results show that the large pre-bend blade exhibits more significant geometric nonlinear behavior during the loading process.Besides,in the case of large deformation,the linear Euler-Bernoulli beam model and Timoshenko beam model will greatly increase the prediction error of the deformation.In addition,the BaMB model can accurately predict the small and large deformations of the wind turbine blades,and the prediction accuracy of the deformation is close to the finite element method,but the efficiency is far beyond the finite element method.Additionally,the ability of the BaMB model to describe the complex geometry and predict the nonlinear deformation of the blade is verified by the results.In this paper,the modal characteristics of the blade are analyzed by combining the BaMB model and the modal parameter identification method.The influence of the dynamic stiffening effect on the natural frequency and operational safety of the wind turbine blades under the rotating state is fully discussed,as well as the influence of pre-bend and gravity on the natural frequency of the rotating blade.The result reveal that pre-bend and gravity have little effect on the natural frequencies of rotating blades,but they will strengthen the coupled vibration of blades in the flapwise direction and the edgewise direction.Additionally,the dynamic stiffening effect will significantly increase the flapwise natural frequency of each order of the blades,and the increase will decrease with the increase of the modal order.Among them,the first-order flapwise natural frequency can increase by about 20%at the rated speed,which leads to the corresponding rotating speed at the intersection of the 1st flap frequency and 3P increases in the Campbell diagram when rotation is considered.Moreover,the significant increase in the flapwise natural frequency narrows the gap between the natural frequency of flapwise direction and the edgewise direction,as well as the natural frequency of flapwise direction and the twist direction.And as the blade flexibility increases,this phenomenon becomes more obvious,which is likely to cause stability problems.Regarding the wind rotor imbalance problem,this paper simulates and analyzes the dynamics response of a 1.5 MW wind turbine under shear wind in different control intervals,in which the wind turbine has additional concentrated mass and discrete mass deviations,as well as the single blade has different pitch errors.This research reveals the influence of the mass imbalance and the aerodynamic imbalance caused by pitch errors on the various parameters of the wind turbine,and provides theoretical guidance for the detection and identification of the rotor imbalance.In addition,this paper comprehensively analyzes the effects of the different pitch errors on the various parameters of the wind turbine under the full wind speed turbulent wind from the cut-in to the cut-out.These parameters include output power,annual energy production loss,pitch angle of blades,loads and deformations of blades,and the axial acceleration spectrum of the nacelle,etc.Based on the analysis results,a method for accurately identifying and positioning pitch errors of wind turbine is proposed,and the method has important guiding significance for the detection and recognition of rotor imbalance in engineering.Furthermore,the judgment indexes of this method are the average wind speed in the measured data,the relative magnitude of the average blade bending moment or deformation,and the 1P/3P spectrum of the nacelle axial acceleration.