Study On Modeling And Torsional Fatigue Loss Of Doubly-fed Wind Turbine Drivetrain

Doubly-fed wind turbines are complex electromechanical coupling systems and the drivetrain of a double-fed wind turbine is an important part of the power transmission of the turbine.Due to the presence of flexible components such as blades and gearboxes,the drivetrain has the characteristics of an underdamping system,so the dynamic changes in the pneumatic torque of the impeller and the electromagnetic torque of the generator can excite the torsional vibration of the drivetrain.Torsional vibration of the drivetrain will not only fatigue the key components of the drivetrain,reducing reliability,but also cause fluctuations in the output power of wind turbines,directly affecting the stability of the grid-connected system.Existing studies show that drivetrain flexibility and electromagnetic torque fluctuations are the most important causes of drivetrain torsion.Therefore,the study of drivetrain flexibility modeling and drivetrain torsional fatigue loss is of great practical significance to suppress the torsional fatigue loss of the drivetrain shaft system,improve the electrical control strategy,drivetrain torsional safety analysis,condition monitoring and life management.This paper proposed a drivetrain flexibility modeling method to study the torsional characteristics of the drivetrain.We proposed an analytical method with theoretical reference value,which combines the characteristics of drivetrain flexibility and electromagnetic torque dynamic perturbation,and used grid short-circuit failure as a typical grid perturbation factor.Then taken the electromagnetic torque load excitation of generator under grid short-circuit failure as the research background,and through theoretical analysis,parametric modeling and simulation experiments,we studied the effects of electromagnetic torque fluctuation on the torsional characteristics of the drivetrain of double-fed wind turbines,the torsional response laws,and the evaluation of the cumulative effects of the drivetrain fatigue loss laws and fatigue damage.The main findings of the study and the results are as follows.(1)Based on the actual structure and geometrical parameters of a 1.5 MW model running on a grid,a multi-flexible drivetrain finite element model suitable for studying the torsional characteristics of drivetrain was established under the ANSYS general finite element platform.A double-fed wind turbine electromechanical coupling simulation model was established under the MATLAB/Simulink platform.The inherent characteristics of the drivetrain were analyzed,the frequency and vibration patterns of the first twelfth order torsional modes were found,and the validity of the proposed model was verified.The concentration of torsional stress in the drivetrain was found through static stress analysis,and the dangerous parts of the drivetrain were mainly on the three drive shafts of the drivetrain.(2)Analyzed the main frequency components of electromagnetic torque generated by short-circuit failure,the transmission intensity of torsional vibration of the drivetrain and transmission laws.The most severe oscillation of the electromagnetic torque of a double-fed generator occurs when a three-phase grounding short-circuit failure occurs.The transmission strength and the transmission law of the torsional vibration of the drivetrain are analyzed by the time domain response of the transmission torque and the relative torsion angle between the mass blocks under short circuit failure.Finally,the torsional response of the drivetrain is analyzed by spectrum analysis of the transmitted torque on the shaft system,and the transmitted torque between the gearbox and the generator rotor can reflect the torsional frequency in the drivetrain.(3)Torsional fatigue damage to the drivetrain is a result of torsional vibration.In the drivetrain,the high-speed shaft is the most susceptible to fatigue loss and has the lowest reliability,and the part where fatigue loss occurs is the same as the torsional part in the first-order torsional mode of the high-speed shaft.The low-frequency component of the electromagnetic torque excites the first-order torsional mode(1.65 Hz)of the high-speed shaft and the high-speed shaft torsional vibration of the drivetrain.The high-frequency component of the electromagnetic torque excites the first-order torsional mode(62.611 Hz)of the intermediate shaft in the gearbox and the solar wheel shaft corresponding to the intermediate gear shaft,which excites the torsional vibration of the intermediate shaft and the solar wheel shaft,resulting in torsional fatigue loss,reducing fatigue life and reliability.Torsional fatigue of the drivetrain mainly occurs in the three drive shafts of the gearbox,and three-phase short-circuit failure has the greatest impact on the fatigue damage to the drivetrain.With the transfer of torsional vibration,the effect of torsional vibration on the parts away from the generator gradually decreases,and the torsional vibration excited by the electromagnetic torque has a transfer effect.(4)Short-term fatigue damage to the drivetrain has a cumulative effect,the electromagnetic torque fluctuation load generated by the grid failure has no significant effect on the drivetrain fatigue loss in the short term,but the cumulative effect of short-term fatigue damage generated by the failure will aggravate the unit drivetrain fatigue loss,causing the cumulative fatigue life of the drivetrain to decline and even fail within the design life.