Electromechanical Coupling Effect In DFIG-Wind Turbine And Active Control For Load Reduction

With the continuous increase in single-wind turbine capacity,disturbance to the power grid is becoming stronger due to the penetration of wind power while greater capability for withstanding power grid fault is required for wind turbine.In the severe situation of power grid and wind excitation,dynamic loads should be limited to a reasonable range for reliability.Therefore,the research on multi-coupling effect in wind turbine and load reduction control is of great significance.From the view of point of dynamic loads and electromagnetic transients,this dissertation aims to give a comprehensive analysis for the electromechanical coupling effect and in-depth study on loads reduction control for the large scale MW-wind turbine based on doubly-fed induction generator(DFIG).The main work and achievements are listed as following:1.In traditional wind turbine model,flexible components,detailed drive train characteristics and electromagnetic transients cannot be considered simultaneously.In order to overcome the exsiting shortcomings,a multi-coupling model is proposed.The model deals with the aerodynamics and mechanical dynamics are modeled in FAST.It also takes account of the drive train and electrical subsystems in Matlab.Compared to the traditional methods,the new drive train model includes more details,such as gear meshing and multi-stage gearbox,meanwhile disturbance with low frequency in planetary gear stage can be analyzed.Aiming at improving stability of solution and enhancing computation speed,grid-side converter and protection circuit for low voltage ride-through(LVRT)are simplified.Additionally,a model is given for offering reference signal during voltage dip in rotor-side converter(RSC)control.The proposed subsystem model and multi-coupling model are verified by experiments,field test and simulation results in different software codes.2.Based on the proposed multi-coupling model,dynamic characteristics and coupling effect of DFIG-wind turbine under normal operation are studied.An investigation is performed on drive train characteristics,and flexibility influence of components on electrical system and electrical effect on dynamic loads are inquired.The key electromechanical coupling relationship is derived in the normal operation situation.Low pass filter characteristic in drive train is founded and the influence of stiffness in drive train on the system is examined.In addition,planetary gear stage disturbance to the system is analyzed on the basis of the practical measurement.According to the simulation results,disturbance with low frequency in drive train has pronounced effect on electrical variables;that with higher frequency has not,however.Flexibility influence of blades and tower on electric power fluctuation cannot be ignored for the strong wind excitation and lower short circuit capacity of power grid.The tower side-side load has a close relationship with electrical system.3.The impact on drive train due to voltage dip is investigated in consideration of RSC control strategies and operation mode of protection devices during LVRT.It is founded that different RSC control strategies and the crowbar investment time have significant effects on drive train.The different perspectives are reflected on dynamic response of wind turbine from electrical and mechanical aspects.The results are useful to the design of wind turbine control strategies.4.In order to study the coupling effect in different components on voltage fault,both qualitative analysis and quantitative simulation are performed,taking account of the different voltage dips and wind excitations.The relationship is highlighted among blade edgewise mode,tower side-side mode and drive train torsional vibration.For further investigation on different impact on tower and blade,peak accelerations on tower top and blade tip are analyzed and a quantitative criterion is given.The results show that blade edgewise mode is mainly affected by the wind turbulence and insensitive to small dip of voltage,which is useful to the determination of control objectives under voltage dip.5.In order to reduce the risk of overspeed under gust and mitigate the impact on dynamic loads caused by voltage dip,control objectives in the special situation are given based on the comparison of load characteristics in different operation cases.Rotor speed stability can be improved by the compensation of pitch angle based on the linear quadratic gaussian and kalman estimator.Also,gain switch method based on the cooperative control of torque and pitch compensation is presented for control objective switch from normal operation to special condition.Moreover,transmission stiffness cannot be determined accurately,and so torque compensation control based on dual kalman filter estimator is proposed for the control optimaztion.Through the theoretical analysis and simulation,the proposed control strategies can be verified.The results show that overspeed risk can be reduced and the impact on the wind turbine can be mitigated effectively.