Integral Support Structure and Controller Design of an Offshore Wind Turbine

Abstract

In the past decade, the wind energy subsidies provided by the Dutch government is declining. In order to adopt wind energy as a well-established source of renewable energy, the costs of (offshore) wind energy should be lowered. This can be achieved by increasing the energy production or by lowering the maintenance and initial costs. In search of lowering the costs, the wind industry is pushing towards the boundary limits of design. The use of multi-objective design is needed for exploring those limits, for which the field of control and structural engineering get together. Currently, little research is conducted of combining controller and structural design in the wind turbine industry. The goal of this thesis is to investigate the possibility of optimizing the support structure design and the controller design into one single optimization routine. The method describes the use of nonsmooth H∞-synthesis. The modelling principles of the turbine makes use of different approaches. A simple finite element model of the wind turbine tower is constructed. The wall thickness of the tower sections is extracted as a tunable parameter, through an affine representation of the Ordinary Differential Equations (ODEs). In this thesis, the Controller Structure Optimization (CSO) for the offshore wind turbine combines the design of a load reducing Rotor Speed Controller (RSC) and wall thickness reduction. In theory, the bandwidth of the RSC is limited by the first tower bending mode. Therefore, the bandwidth of the designed controller is chosen well below the first eigenfrequency. It was found that the limitations do not influence the solution. The limitations of the proposed method are bounded by the weight functions, which impose a limit on the performance. Furthermore, it was found that the CSO framework can minimize the wall thickness of the tower elements, and simultaneously design a suitable RSC for tracking a rotor speed. The results are verified in high-fidelity wind turbine model, where the control-relevant subjects are addressed and specified. It was found that by addressing multi-objectives, we can design a controller with rotor tracking abilities and load reducing properties. Hence, the proposed CSO framework, can simultaneously design controllers and alter the structure and thereby increase the overall performance of the system.