A study of the dynamic response of wind turbine gearboxes

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Gearbox is an important component for large modern wind turbines incorporated either by a squirrel cage induction generator or a doubly fed induction generator. Wind turbine gearboxes have distinct features from standard gearboxes. They are used to increase the rotor speed to a speed suitable for the electricity generation and operate under varying load conditions, while standard gearboxes are designed to step down from high speed to low speed and operate under full load conditions. The modern wind energy industry has been experiencing high gearbox failure rates since its inception. However, the fundamental mechanisms of gearbox failures have not been fully understood yet. Thus, this thesis studies the dynamic response of wind turbine gearbox components in order to provide useful information to the wind energy industry to reduce the possibility of the gearbox failures at an early stage. The torsional vibrations of wind turbine gearbox are firstly investigated in this thesis. The nonlinear dynamic model developed considers the factors such as time-varying mesh stiffness, damping, static transmission error and gear backlash. Both the external excitation due to wind gust and the internal excitation due to static transmission error are included. With the help of time history, FFT spectrum, phase portrait, Poincare map and the effects of the static transmission error, mean-to-alternating force ratio and time-varying mesh stiffness on the dynamic behaviour of wind turbine gearbox components are investigated by using the numerical integration method. It is found that the external excitation has the most influence on the torsional vibrations of the wind turbine gearbox components. The gear mesh stiffness has more influence than the static transmission error, and the static transmission error has the least influence. Secondly, the dynamic response of a proposed four-degree-of-freedom (4DOF) wind turbine gearbox dynamic model is studied. The effects of different excitation conditions are discussed. The results show that the external excitation fluctuation has large influence on the dynamic responses of both the gears and bearings, and explain under which conditions the fretting corrosion, as one of the wind turbine gearbox failure modes, may occur. Thirdly, the effects of bending moments on the dynamic responses of a wind turbine planetary gearbox are analysed. The proposed six-degree-of-freedom (6DOF) dynamic model takes into account the key factors such as the time-varying mesh stiffness, bearing stiffness, damping, static transmission error, gear backlash and bearing clearances. It is found that the bending moments can affect the gear meshes. What is more, the driving torque may have the effect on the bending moments. Furthermore, the bearing clearance has negligible effect in the planetary gear stage.
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