Research On Pitch Control Method For Multi-Objective Coordinated Optimization Of Large Offshore Wind Turbines

The main function of the wind turbine control system is to ensure efficient operation,stabilize the output power,reduce the structural load,and guarantee operation and survival safety.With the rapid development of large offshore wind turbines,the current pitch control method is difficult to meet the reliability design requirement.Therefore,it is urgent to study the optimization method of pitch control.The technical route is theoretical derivation,mechanism analysis,control optimization,and simulation analysis,which is based on a 5MW fixed offshore wind turbine.The multi-objective coordinated optimization of power,load,and safety of large offshore wind turbines is put forward.The research is carried out with the support of the National Key R&D Program Project "Optimization design,batch manufacturing process and testing technology of key components of drivetrain" for large offshore wind turbines(2018YFB1501304).Research works and achievements are as follows:(1)Adopting the dynamic analysis method,the mathematical model of the offshore wind turbine is derived and established based on the coupling characteristics of rotor rotation and tower motion under aerodynamic force,which is suitable for the design of the collective pitch controller.The wind disturbance and wave disturbance are suppressed comprehensively under the stochastic disturbance accommodating controller.The collective pitch controller is studied to optimize the tower load and reduce the output power based on the linear quadratic regulator.It is shown in the simulation that the standard deviation of output power is optimized by 39.16%and the tower fore-aft fatigue load in the hub coordinate system is reduced by 12.34%.(2)It is proved that the transfer function from pitch angles to root flap-wise moments of three blades cannot be decoupled in a fixed coordinate system under Coleman transform,while the traditional individual pitch controller with the proportional-integral structure cannot handle the coupling characteristics.Therefore,the multivariable control theory based on the state-space model is applied to optimize the individual pitch control method.The two degrees-of-freedom robust individual pitch controller is put forward to balance the problem between multi-objective tracking and wind disturbance suppression.The customized design of closed-loop system bandwidth is realized by the reference model method.It is shown in simulations that blade root flap-wise fatigue load,tower fore-aft fatigue load,and tower side-to-side fatigue load are reduced by 18.68%,32.92%,and 6.22%,respectively,without affecting the output power.(3)The response characteristics of the traditional individual pitch control method are explored under mass imbalance and aerodynamic imbalance based on the time domain simulation method.The failure mechanism of the proportional-integral control method under aerodynamic imbalance is revealed based on the frequency domain analysis method.The compensation problem of aerodynamic imbalance above rated wind speed is solved by optimizing the two degrees-of-freedom robust individual pitch controller.It is shown in simulations that different aerodynamic imbalance faults are compensated completely by the proposed method.Compared with the extended Coleman transform method,the blade load and tower load increased by aerodynamic imbalance are optimized by the proposed method.In the simulation,the 1P component of the blade flap-wise moment and the 0P component of the tower side-to-side moment are reduced by 21.22%and 6.84%,respectively.(4)Given the lack of stability and adjustment ability of the classical peak shaving regulator,the peak shaving regulator based on model predictive control is proposed.The rapidity and stability of response are ensured by the feedback control framework based on the open-loop solution of the closed-loop optimization problem.The online control of the pitch angle range for safe operation is realized by the quadratic programming algorithm based on the linear state-space model of wind turbines.When it is near the rated wind speed,it is demonstrated that the hub thrust is protected in a safe range under both turbulent wind and gust.Moreover,the rotor ultimate load is reduced by 14.17%.The hub fatigue load,blade fatigue load,and tower fatigue load are reduced by 20.04%,6.58%,and 1.27%,respectively.Although the power loss is 0.79%,operation safety is ensured under dangerous conditions such as high wind speed,turbulence,and gust.(5)The response characteristics are studied on rotor speed,tower bending moment,and blade bending moment during shutdown with different pitch rates.A closed-loop shutdown method based on linear active disturbance rejection pitch control is proposed by taking the generator speed and the tower fore-aft speed as the control objectives,which can adjust the pitch rate dynamically.The simulation results under three fault conditions,including yaw misalignment,gird loss,and blade seizure,show that the comprehensive optimization of wind turbine response is realized by the proposed control method during the shutdown,such as alleviating overspeed,limiting tower fore-aft movement,and weakening impact load.