| In order to capture more wind energy,the structural size of wind turbines is gradually becoming larger,leading to more flexible structures.For offshore wind turbines,they not only suffer wind and wave loadings,but also may be subjected to seismic loadings when they are installed in earthquake-prone areas.The eismic loading may be another important source of structural vibration in the life cycle of the wind turbines installed in earthquake-prone areas.Therefore,it is necessary to control the structural vibration of offshore wind turbines to ensure their safe operation.Moreover,related studies also show that the establishment of a detailed dynamic model of wind turbines is necessary to accurately evaluate the dynamic response of wind turbines and the effectiveness of structural vibration control methods.Based on the above analyses,the main research objectives of this paper are as follows:(1)Establishing an aero-hydro-elastic-control coupled simulation model of a monopile offshore wind turbine.In order to simulate the performance of wind turbine with high precision,three kinds of multi-body gearbox models with different complexity,namely the torsional gearbox,the six-degree-of-freedom gearbox and the flexible gearbox,are established based on multi-body dynamics modeling technique.They are then compared and analyzed to provide the basis for detailed modeling of the drivetrain and whole wind turbine model.The results showed that the torsional gearbox model has the advantages of short calculation time and low cost,and can be used for preliminary qualitative analysis.If more detailed and accurate dynamic characteristics need to be described,more freedoms and flexibilities should be introduced into the drivetrain model.Based on the dynamics model of the wind dynamic including blades,towers and drivetrains,combined with the seismic excitation model,hydrodynamics,aerodynamics and pitch-torque control system,a aero-hydro-elastic-control coupling simulation model of the monopile offshore wind turbine under wind/wave/seismic excitations is established.The establishment of the coupled wind turbine model provides a platform for the following simulaitons.(2)Analyzing the dynamic response characteristics of the monopile offshore wind turbine under multiple excitations and conditions.Based on the coupling simulation model,the influence of pile-soil interaction on the dynamic response of the wind turbine under normal operating conditions and parked conditions is studied.Simulation analysis under wind-wave excitations and wind-only excitation is carried out considering pile-soil interaction.The coupling analysis model of the wind turbine under wind-wave-seismic excitations is established,and the simulation analysis of wind-wave excitation and wind-wave-seismic excitation is carried out.The results showed that pile-soil interaction mainly affects the tower-base loads,and its effect is greater under the parked condition than under the operational condition.In addition,the wind turbine system shows nonlinear and almost linear dynamic characteristics under operational and parked conditions,respectively.It can also be found that the seismic excitation can significantly affect the dynamic response of the wind turbine,and the aerodynamic damping plays an important role in the influence of the seismic excitation on structural response of the wind turbine.(3)Using individual pitch control strategy to control structural vibration of the monopile offshore wind turbine.In order to solve the impact loads caused by the large pitch rate and the lag pitch response near the rated wind speed,a pre-pitch control strategy is designed to make the smooth transition pitch angle near the rated wind speed so as to reduce the impact loads on the wind turbine.Taking the maximum structural load as the index,the load reduction effect of the pre-pitch control approach is evaluated.The simulations indicated the effectnivess of the the pre-pitch control method.Aiming at the antisymmetric loads of the rotor caused by the vertical wind shear effect,a disturbance generator and a state estimator are designed based on the linearization of the wind turbine model,and a disturbance adaptive rule is determined.The linear quadratic regulator(LQR)is appled to determine the state gain and estimator gain.Considering the wind shear effect,the dividual pitch control strategy is designed.Then,the final individual pitch controller is proposed combined with the pre-pitch collective pitch controller.Not only the unbalanced loads on the rotor caused by the wind shear effect are suppressed,but also the output power is maintained near the rated value.The results showed that the designed individual pitch control strategy can not only effectively suppress the 1p loads,but also mitigate other key loads.Moreover,the individual pitch control strategy can reduce the fluctuation of rotor speed while keeping the rotor speed near the rated value.(4)Applying passive tuned mass damper systems to suppress structural vibration control of the monopile offshore wind turbine.Based on the modal analysis results of wind turbine system and the assumption that the mass and damping ratio of each TMD are equal,the parameters of the TMD system are calculated based on the formula method to restrain the dynamic response of the wind turbine under wind-wave excitations.Four systems with different TMD numbers in the nacelle,namely the single TMD,2-MTMD,4-MTMD and 10-MTMD,are designed.Based on the wind turbine coupled TMD system model,the response analysis of the wind turbine under wind-wave excitations with TMD control is carried out for multiple operating conditions.Based on the failure hypothesis of the TMD system,the vibration control robustness of the 10-MTMD system is studied.The results showed that due to the nonlinear characteristics of the wind turbine under the operational conditions,the vibration suppression effect of TMD is more significant under the parked condition.In addition,the 10-MTMD system showed remarkable robust performance,especially in the parked condition.Based on the dynamic response characteristics of the wind turbine structure under wind-wave-seismic loads,the results of modal analysis and the assumption that the natural frequency of TMD system is uniformly distributed near its average natural frequency and that each TMD has the same mass and damping coefficient,the simplex coding genetic algorithm is applied to optimize the damping and stiffness coefficients of the single TMD system and the multiple TMDs system,where the minimum mean square value of structural displacement is taken as the optimization objective.Three systems with different TMD numbers are designed,including the single TMD,2-MTMD and 10-MTMD.Based on the wind turbine coupled MTMD system model,the response analysis of the wind turbine under wind-wave-seismic excitations with TMD control is carried out under multiple operating conditions.The results showed that the MTMD system can effectively suppress the tower loads under the earthquake excitation.The spring coefficient and damping coefficient in the pile-soil interaction model are changed respectively,and the robust characteristics of vibration control of the10-MTMD system under wind-wave-earthquake excitations are studied.Meanwhile,the influence of the change of parameters in the pile-soil interaction model on the dynamic response of the wind turbine is studied using the frequency response method.The results showed that the 10-MTMD system exhibits robustness when the soil parameters are changed,especially when the soil damping constant is changed. |
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