COMPACT AND ROBUST HIGH EFFICIENCY SHOCK-AND-ERROR-TOLERANT MECHANICAL POWERTRAINS FOR WIND TURBINES APPLICATION

Abstract

Large forces and torques experienced by both small and large wind turbines influence the deflection, loading and overall dynamical response of the gearbox and other powertrain elements, often leading to failure in the drivetrain components. Gearbox-related failures, caused by shocks and misalignments due to gearbox component deflections, generator/grid engagements, etc, are accountable for more than 20% of the wind turbine downtime, resulting in high operational expenditure (OPEX). To address this problem, this thesis presents three main axes of innovation that we propose to be essential for further development of wind turbine technology. The first two parts will introduce novel technologies that are meant to provide an improved wind turbine powertrain system that is robust, compact and efficient. Finally, a material hysteresis model is introduced for system dynamics. This thesis envisions an architecture that will resolve existing limitations constraining the evolution of wind turbine technology and act as an enabler for the vision of possible future wind turbine technologies. First, the substitution of contemporary multi-stage gearboxes in the wind turbine with a more lightweight, compact, robust, and reliable cycloidal drive (speed increaser) of novel design. Traditional transmission gears such as spur or beveloid gears are said to have limitations in power and sizing, and therefore their common alternatives are planetary gears, such as cycloidal gears which have a single-stage transmission ratio. However, current available cycloidal drives, employ a cycloidal teeth profile, which leads to the primary limitations due to dimensional tolerances such as torque ripple and backlash, affecting its efficiency. A possible novel solution is the use of a cycloidal drive fitted with involute teeth mesh. Proper implementation grants superior performance in terms of constant pressure angle and invariance of velocity ratio (more than 1:100) with respect to centre distance alteration. The assessment of these drives is conducted via finite element (FE) and dynamical multi-body analysis, where the stresses, contact forces, torque ripple and efficiency are evaluated. Furthermore, to mitigate the impact of shock on the system, especially in the gearbox, a novel hybrid clutch is introduced that fuses the concepts of dog clutches and disc clutches currently available in the industry. The idea of a hybrid clutches is to couple two rotating shafts by a form-type interface to provide maximum torque/volume ratio, while the disc clutches which are already in use in wind turbine systems have lower torque/volume ratio and are best at absorbing shock transients and used to mitigate the short-term torque peaks by slipping at a defined maximum torque in order to briefly interrupt the drive. This clutch can be placed between the generator and the cycloidal drive. The proposed clutch will have a graded stiffness distribution, achieved by a complex internal topology, so that at low torques and during transients it will present a sliding disc surface interface, whereas at higher loads, by increasing the compressive force between the clutch discs, these will be deformed to a mating ‘wavy’ form-type interface (the equivalent of a dog clutch), characterized by a much higher load carrying capacity, thereby securing high efficiency at high loads. Analysis of this design is done with finite elements to examine its load-carrying capacity and dynamical response. Additionally, mechanical and structural systems' damping behaviour is often described using viscous damping models like the Voigt-Kelvin model. These models can be helpful, but they may not be adequate for analysing complicated structural systems. For this reason, a generalised novel finite element non-linear hysteresis model is developed from the modification of the conventional Kelvin-Voigt model, to produce a non-viscous hysteretic behaviour. The MDOF hysteresis model is based on its instantaneous state under forced vibration and does not need previous information on the history of motion, excitation parameters, or frequency. Currently, the finite element packages do not yet have any damping estimate techniques that are both accurate and reliable. Therefore, such a robust model is required that will yield reliable results regardless of the excitation frequency and will not need special calibration of the model parameters, as the Kelvin-Voigt family of models, including beta-damping, requires. In the design of gears and gearboxes, damping models may be utilised to improve the design, which in turn results in lower levels of vibration as well as noise.