Optimal Control for Fatigue Reduction of a Ballast-Stabilized Floating Wind Turbine

Abbate, Giuseppe Battista

4. term

Control and Automation, Master

2013

2014-01-07

34 pages

The increasing demand for renewable energy all the world led to a development in wind power technologies. Thus, wind turbines are increasing in dimension and power production. So far they are mounted on land or in shallow waters with basement. The main disadvantage is that wind turbines have a visual impact that sometimes cannot be accepted. Also, because of the aerodynamic interaction between the wind and the blades, they cause ambient noise near the structure. Hence, the objective during the last years is to install these structures in deeper water, where there’s no visual impact and the ambient noise is not relevant. There’s also another advantage for going further offshore into deeper waters: wind is less turbulent due to the fact that there are no mountains, buildings or something else that can deviate the wind flow. The unique economically favourable way to move in deeper water is using floating platform. Fortunately, floating platforms were deeply studied by the oil and gas companies for their oil plants, and these studies have also been used for the wind turbines. However, using a floating wind platform introduces additional motions that must be taken into account in the design stage. Therefore, the control system becomes an important component in controlling these motions. It has been shown that using the onshore baseline controller (PI blade pitch controller plus variable speed generator torque controller) can cause the problem of negative damping in the offshore wind turbine. The first part of the report aims to describe briefly the tuning procedure of the baseline controller and the development of a simplified control-oriented model of offshore floating wind turbine for a ballast stabilized platform. The analysis focuses on the dynamics of a wind turbine, the development of a model starting from the forces that act on the system, the identification of the parameters and finally the validation of the model using the detailed wind turbine simulator FAST, freely available online and accessible in the literature (1). The second part of the report describes three types of control techniques for the wind turbines, based on both classic and advanced control theory. It has been used a baseline PI controller and an three LQ controller with different objectives. The purpose is to apply and to analyze a control law that aims to reduce the fatigue in the tower. The main idea was to minimize the variance of the tower deflection or the variance of the tower deflection velocity. The results showed that with the LQ controllers, reducing the tower deflection velocity, it is possible to reduce the fatigue in the tower base. The last part of the report consists in a comparison between the three LQ controller described before and the baseline PI, based on the Damage Equivalent Loads and on a statistic analysis. Each controller will be used in simulation with FAST in order to understand if the control law actually minimizes the fatigue and legitimates the use of an advanced controller.

hywind ; fatigue ; control ; floating wind turbine ; LQ ; linear quadratic

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