Research On Running Characteristics And Control Of Doubly-fed Induction Generator Under Blade Imbalance Fault

Wind energy has become one of the most competitive new energy resources in the world wide because it has the advantage of clean,green and no pollution,thus wind power generation technology becomes the key research subject throughout the world.With the development of the wind power generation technology,installed capacity and the blade length get rapid increase as well,resulting in the unit drive train damping decrease and flexibility increase.Thus the influence of the aerodynamic loading variation on the wind turbine generation system(WTGS)becomes more complex and difficult to control.So it is important to research on the aerodynamic characteristics and the influence on the WTGS and its control strategy when aerodynamic loading varies.Blade imbalance fault,which is the common fault of wind turbine,is one of the key factors that affect the aerodynamic characteristics of the wind turbines.At present,the research about the blade imbalance fault is not comprehensive enough.Especially,there are seldom researches about the influence on the generator of WTGS and its control strategy.The diagnosis mechanism of the fault is not consummate.So this thesis carries out the systematic study on the blade imbalance fault for WTGS with doubly-fed induction generator(DFIG).Firstly,the common fault types and the fault mechanism are analyzed,and the DFIG electrical characteristics under the fault are discussed as the key problem of the paper.Then the Maximum Power Point Tracking(MPPT)control strategy and its cross coupling with the fault are analyzed in detail.Finally the improvement control strategy is proposed to reduce the impact of the fault on the generating efficiency and the power quality.The detailed research contents are given below:(1)Set up the simulation platform suitable for the blade imbalance fault analysis.This platform includes the wind turbine aerodynamics model,blade element moment(BEM)theory,structural dynamics of the nacelle and tower,drivetrain model of the WTGS,transient characteristics and the vector control system of the DFIG.Using this platform we can carry out the simulation of the blade imbalance fault and the DFIG characteristics.(2)Blade imbalance fault mainly includes blade mass imbalance and the aerodynamic asymmetry.The mass imbalance fault mechanism is firstly analyzed.An analysis method based on the drive train model and dq coordinate system transformation is proposed to obtain the characteristic of the DFIG under the mass imbalance fault.Then the simulation and experiment test of blade imbalance fault are taken to verify the theory analysis.(3)First the blade aerodynamic asymmetry fault mechanism is analyzed.The blade airfoil profile is changed because of the aerodynamic asymmetry,so that the aerodynamic loading varies from the normal condition.Then the new concepts of additional axial aerodynamic thrust,additional bending moment,additional tangential aerodynamic force and the additional aerodynamic torque are put forward.Those additional items make the blade,tower and nacelle vibration,which will change the wind speed relative to the blade element and further change the aerodynamic loading and the DFIG running characteristics.For the first time,the DFIG stator and rotor characteristics under aerodynamic asymmetry fault are obtained.A fault diagnosis method based on the current and power is put forward.(4)Discuss the two modes of MPPT control based on optimal power–speed curve,and analyze the intersection and coupling between MPPT control and the blade imbalance fault.Combining with the DFIG rotor side converter vector control(RSC-VC)strategy,the influence on the DFIG electrical characteristic are analyzed again and some new conclusions differently from the former researches are drawn.(5)Blade imbalance fault results in the aerodynamic loading periodic fluctuation,evenly decreasing in large degree,which will aggravate the vibration and further affect the lifetime of the WTGS.Beside,the power quality and power generation efficiency will decrease as well.These problems have great relation with the characteristic of lower damping and high flexibility of drive train because of the large size of modern WTGS.By tracking the source of aerodynamic loading,the damping characteristic of the unit is analyzed based on the drive train equivalent model,and torque compensation control strategy is proposed to the increase the unit damping.Using this method,the aerodynamic loading oscillation and the harmonics in the electrical quantity(current and power)are suppressed in large degree,thus the power quality is enhanced.The thesis systematically researches on the blade imbalance fault mechanism and the influence on the DFIG running characteristics,the coupling with the control system and the improvement control strategy.The research conclusions make the fault theory system of WTGS with DFIG more consummate,and it has practical significance to reduce the unit vibration,enhance the unit lifetime and power generating efficiency,and reduce power generation cost.