Research On PMSG-based WECS Vibration Suppression And Robust Control

In recent years,although wind power generation has achieved considerable development and progress at the technical level,there are still many problems to be further studied and discussed,such as how to suppress the vibration of wind turbine and how to enhance the robustness of its control system.Due to the randomness and uncertainty of wind speed,tower shadow effect,wind shear and other factors,the vibration of wind turbine always accompany the entire process of the unit's operation,which will not only produce huge pollution noise,but also easily cause the loosening of the unit's parts,damage to the actuators and seriously affect the service life of the unit.The vibration of PMSG-based WECS mainly involves the torsional vibration of drive train,the vibration of tower,the flutter of blades and the possible coupled vibration between them.The torsional vibration of drive train will not only cause damage to the gearbox,shaft coupling and other shaft damage,but also may cause the problems of SSO(sub-synchronous oscillation)and SSH(sub-synchronous harmonic).Meanwhile,Long-term tower vibration and blade flutter will cause fatigue of tower and blades,which may cause many accidents such as the tower collapse and blades broken.Furthermore,there are some uncertainties in the actual operation and control process of PMSG-based WECS,such as the parameters perturbation caused by the change of generator room temperature,the unmodeled power converter dynamics and the torque fluctuation caused by the rapid change of wind.These uncertainties may affect the efficient and stable operation of the unit.The closed-loop system must have strong robustness to ensure the stable operation of PMSG-based WECS.However,due to the problems of difficult parameter tuning and poor robustness of conventional control,it is sometimes difficult to meet the control requirements.Therefore,it is also necessary to study the robust control scheme for unit uncertainties.This paper focuses on the vibration suppression and robust control of PMSG-based WECS,and has achieved some research results as follows:Firstly,the general mathematical model of PMSG-based WECS was established.Its basic structure,operation mechanism and basic control strategy were introduced in detail.The excitation source of wind turbine vibration and the uncertainties of wind turbine were deeply analyzed.The basic concepts such as"turbulent flow","tower shadow effect","wind shear effect" and "wind turbine stalling" were delineated,and their hazards to wind turbine were also carefully analyzed.Secondly,the torsional vibration mechanism of PMSG-based WECS under optimal tip speed ratio(OTSR)control was analyzed.Through theoretical derivation,it is concluded that the change of capacitor energy storage contains the information of torsional vibration.From this point of view,a torsional vibration suppression scheme that introduces compensation torque and compensation speed related to capacitor energy storage in the torque loop and the speed loop was proposed to effectively suppress the torsional vibration under OTSR control.Thirdly,based on the central manifold theorem,the stability of PMSG-based WECS under pitch control based on speed feedback(PCBSF)+direct power control(DPC)was analyzed,and the system model was also reduced.The torsional vibration mechanism under PCBSF+DPC was analyzed based on the reduced-order system model.Then,the torsional vibration suppression method with additional stiffness and damping is proposed.Compared with the conventional torsional vibration suppression method,the method proposed in this paper not only has an excellent suppression effect on the high-frequency torsional vibration at the natural frequency of the drive train,but also can effectively suppress the low-frequency forced torsional vibration caused by the change of wind speed.Fourthly,the SSO and SSH problems caused by torsional vibration were analyzed in depth.The research shows that power feedback control will weaken the system damping.When the total damping of the system becomes negative,the torsional vibration of drive train will be unstable due to the occurrence of SSO.In addition,when the system is under-damped,it is easy to generate a large number of SSH in the grid-connected currents and voltages,due to the disturbance wind and controller switching,which will impact the grid and may affect the safety of the entire power system.Fifthly,the detailed mechanism modeling of tower vibration were carried out,and the influence of tower shadow effect and wind shear on tower vibration were analyzed.It is pointed out that the 3P component formed by tower shadow effect and wind shear is still the main excitation source that causes tower vibration,which will excite the tower resonance at a specific speed,causing serious harm to wind turbine.Therefore,a resonance zone crossing method for avoiding tower resonance and the damping control method for suppressing tower vibration were studied.Meanwhile,a more excellent sliding mode pitch control strategy was proposed.Sixthly,The mechanism modeling of blades flutter were carried out,and the effects of tower shadow and wind shear on blades flutter were analyzed.A variety of schemes for suppressing blade flutter including damping control,LQG control,H? control and LQG/LTR control were studied,and the specific process of controllers design was given.The results show that the LQG/LTR control based on independent pitch has a good suppression effect on the blades flutter,which provides us with a certain theoretical support for selecting the control algorithm in practice.Finally,in view of the uncertainties of PMSG-based WECS,the corresponding robust sliding mode control strategies were proposed,which could significantly enhance the robustness of the system.Compared with conventional control,the proposed robust sliding mode control scheme has good robust performance against the uncertainties of the system,which can reduce the distortion rate of current harmonics and the torque ripple.