Fault-tolerant load reduction control for large offshore wind turbines

Offshore wind turbines suffer from asymmetrical loading (blades, tower etc.), leading to enhanced structural fatigue. As well as asymmetrical loading different types of faults (pitch system faults etc.) can occur simultaneously, causing degradation of load mitigation performance and enhanced fatigue. Individual pitch control (IPC) provides an important method to achieve mitigation of rotor asymmetric loads, but this may be accompanied by a resulting enhancement of pitch movement leading to increased possibility of pitch system faults, which negative effects on IPC performance.This thesis focuses on combining the fault tolerant control (FTC) techniques with load reduction strategies by a more intelligent pitch control system (i.e. collective pitch control and IPC) for offshore wind turbines in a system level to reduce the operation & maintenance costs and improve the system reliability. The scenario of load mitigation is analogous to the FTC problem because the action of rotor/tower bending can be considered as a fault effect. The essential concept is to attempt to account for all the "fault effects" in the rotor and tower systems which can weaken the effect of bending moment reduction through the use of IPC.Motivated by the above, this thesis focuses on four aspects to fill the gap of the combination between FTC and IPC schemes. Firstly, a preview control system using model predictive control with future wind speed is proposed, which could be a possible alternative to using LiDAR technology when using preview control for load reduction. Secondly, a multivariable IPC controller for both blade and tower load mitigation considering the inherent couplings is investigated. Thirdly, appropriate control-based fault monitoring strategies including fault detection and fault estimation FE-based FTC scheme are proposed for several different pitch actuator/sensor faults. Furthermore, the combined analysis of an FE-based FTC strategy with the IPC system at a system level is provided and the robustness of the proposed strategy is verified

Passive load alleviation by morphing blades for tidal turbines

This thesis presents a novel passive load control to reduce the unsteady hydrodynamic loads on the blades of tidal turbines, enabling lighter, more resilient and less expensive turbines. Existing load control strategies are inadequate to mitigate the high frequency loads experienced by tidal turbines. The ideal solution should pro vide a fast, local control action on every section of the blade. Research on wind turbines suggests that the most promising option are trailing edge flaps, and passive, flexible materials are recommended over rigid control surfaces to maximise the device reliability. Beyond some pioneering examples of morphing blades, the fundamental principles underlying their efficacy have yet to be fully understood, withholding further development and the adoption by the industry. I present a numerical investigation of morphing blades to show the principles underlying unsteady load alleviation by morphing blades, and I demonstrate their capabilities via proof-of-concept experiments. I develop a low-order model where the blade flexibility is represented by a torsional spring that controls the blade pitch motion, and I optimise the system for a specific turbine design operating in different flow conditions. The fluctuations of the root bending moment can be reduced up to 99% when the turbine operates in shear flow, and by 87% when operating in large wave conditions. The system is governed by the blade flexibility, but the blade inertia, material damping, and unsteady flow phenomena can affect the load-alleviating performance greatly. To verify the system capabilities, I conduct a series of experiments in FloWave, a 25 m wide, 2 m deep, circular tank testing facility, using a 1:15 scale turbine and custom designed passively-pitching blades. The system consistently reduces the fluctuations of the root bending moment, thrust and torque over a range of different tip speed ratios. While the experiments featured a passively-pitching blade, the results are a good indication of the potential of morphing blades, and the analytical low-order code is equally representative of a rigid blade with a flexible trailing edge. This work aims to underpin the future development of morphing blades by providing a simple, yet reliable numerical model, and by proving experimentally the capabilities of this technology

Study of the AFC technology development: a case study applied to wind turbines

The desire to harvest more energy has pushed the contemporary wind turbines to increase their blade spans in order to be able to harness more power from the wind resource. The objective of the following document is to analyze the challenges large scale wind turbines face, and to obtain active flow control solutions that are able to improve their aerodynamic, aeroacoustic and structural performance. To do so, this paper focuses on first understanding the adversities common to wind turbine performance and study their core sources to be able to solve the hindrances with active flow solutions. These are mainly derived from the unpredictable nature of the wind itself, like rapid wind direction and velocity variations, causing aerodynamic phenomena that limit wind turbine’s proper performance. These AFC technologies are then researched and discussed, and from the already marketed systems at Technical Readiness Level 9 Air Jet Vortex Generators were picked for their stall and fatigue reduction capabilities together with the annual energy production increase they are proven to provide. To test the implementation and the economical and environmental impact of AFC systems on wind turbine a script model was developed to serve as a guide for the viability of the project depending on the initial cost of investment. Variables used for the model are gathered through the bibliographic research done in AFC systems together with current programs that pursue our very same goal. This tool will aid in the development of the AFC product in order to accomplish the climate goals our society is heading albeit from a sensible economical standpoint

Wind Turbine Adaptive Blade Integrated Design and Analysis

This project aims to develop efficient and robust tools for optimal design of wind turbine adaptive blades. In general, wind turbine adaptive blade design is an aero-structure coupled design process, in which, the evaluation of aerodynamic performance cannot be carried out precisely without structural deformation analysis of the adaptive blade. However, employing finite element analysis (FEA) based structural analysis commercial packages as part of the aerodynamic objective evaluation process has been proven time consuming and it results in inefficient and redundant design optimisation of adaptive blades caused by elastic-coupled (bend-twist or stretch-twist) iteration. In order to achieve the goal of wind turbine adaptive blade integrated design and analysis, this project is carried out from three aspects. Firstly, a general geometrically linear model for thin-walled composite beams with multi-cell, non-uniform cross-section and arbitrary lay-ups under various types of loadings is developed for implementing structural deformation analysis. After that, this model is validated by a simple box-beam, single- and multi-cell wind turbine blades. Through validation, it denotes that this thin-walled composite beam model is efficient and accurate for predicting the structural deformations compared to FEA based commercial packages (ANSYS). This developed beam model thus provides more probabilities for further investigations of dynamic performance of adaptive blades. Secondly in order to investigate the effects of aero elastic tailoring and implanting elastic coupling on aerodynamic performance of adaptive blades, auxiliary software tools with graphical interfaces are developed via MATLAB codes. Structural/material characteristics and configurations of adaptive blades (i.e. elastic coupling topology, layup configuration and material properties of blade) are defined by these auxiliary software tools. By interfacing these software tools to the structural analysers based on the developed thin-walled composite beam model to an aerodynamic performance evaluator, an integrated design environment is developed. Lastly, by using the developed thin-walled composite beam model as a search platform, the application of the decoupled design method, a method of design of smart aero-structures based on the concept of variable state design parameter, is also extended

Optimal design for a composite wind turbine blade with fatigue and failure constraints

The search for more efficient and sustainable renewable energies is rapidly growing. Throughout the years, wind turbines matured towards a lowered cost-of-energy and have grown in rotor size therefore stretched the role of composite materials that offered the solution to more flexible, lighter and stronger blades. The objective of this paper is to present an improved version of the preliminary optimization tool called Co-Blade, which will offer designers and engineers an accelerated design phase by providing the capabilities to rapidly evaluate alternative composite layups and study their effects on static failure and fatigue of Wind turbine blades. In this study, the optimization formulations include non-linear failure constraints. In addition a comparison between 3 formulations is made to show the importance of choosing the blade mass as the main objective function and the inclusion of failure constraints in the wind turbine blade design. La recherche pour des énergies renouvelables plus efficaces et durables est en forte croissance. Au fil des années, les éoliennes ont acquis de la maturité avec un coût plus réduit et des tailles de rotor plus grandes élargissant ainsi l’utilisation des matériaux composites qui offrent plus de flexibilité, plus de légèreté et plus de solidité. L’objectif de cet article est de proposer une version améliorée du logiciel d’optimisation préliminaire Co-Blade, qui permettra aux concepteurs d’accélérer la phase de conception des pales d’éolienne en matériaux composites grâce à des outils d’études de diverses configuration des laminés composites et de leur comportements en rupture et en fatigue. Dans cette étude, les formules d’optimisation tiennent compte des contraintes de ruptures non linéaires. Additionnellement, une comparaison de 3 formules d’optimisation a été effectuée afin de mettre en évidence l’importance du choix de la masse tel que fonction objective principale et de la considération des contraintes de rupture dans la conception des pales d’éoliennes

Floating Offshore-wind and Controls Advanced Laboratory Program: 1:70-scale Testing of a 15 Mw Floating Wind Turbine

This thesis presents results from the Floating Offshore-wind and Controls Advanced Laboratory (FOCAL) Experiment which performed 1:70-scale model testing of a 15 mega-watt floating offshore wind turbine (FOWT) at the University of Maine’s Alfond Wind/Wave Ocean Engineering Laboratory (W2). The experimental campaigns supported the application of controls co-design to FOWTs by incorporating real-time wind turbine control and hull mounted tuned-mass damper (TMD) elements. A performance-matched scaling strategy was proposed and analysis in OpenFAST performed to design the scale models, including integration of the Reference OpenSource Controller (ROSCO) and modeling of the TMDs in the hull. Four test campaigns were conducted, first characterizing the performance of the turbine and hull separately in wind and wave only environments, respectively. The combined system was then characterized in wind and wave environments, investigating effects of control strategies such as thrust peak shaving, floating feedback, and different TMD frequencies. Data were collected and used to validate OpenFAST models developed by the FOCAL team as well as mid- and high-fidelity tools developed by other ATLANTIS teams. Key outcomes consisted of demonstrating the ability to tune ROSCO and control the scale model turbine and investigating the impacts of ROSCO on system behavior, including attenuation of platform pitch motion and tower base bending moments in response to the thrust peak shaving and floating feedback control. The TMDs demonstrated effective attenuation of platform pitch and tower bending moments at their respective resonant frequencies, as well as synergistic performance when combined with ROSCO’s floating feedback control. These findings identified opportunities for further optimization of FOWT systems through controls co-design and areas of future work to advance ROSCO and scale model testing of FOWTs with controls. Three datasets were generated and uploaded to the Atmosphere to Electrons portal including time history data, technical reports describing the experimental setup and test articles, and numerical models in Bladed and OpenFAST. Ten publications from the FOCAL team describe the design of the scale model, results of the numerical model validation, and discussion of the experimental results including the effects of ROSCO and TMDs on the performance of the floating wind turbine system