This thesis investigates the computation of invariance kernels for planar nonlinear systems with one input, with application to wind turbine stability. Given a known bound on the absolute value of the input variations (possibly around a fixed non-zero value), it is of interest to determine if the system's state can be guaranteed to stay within a desired region K of the state space irrespective of the input variations. The collection of all initial conditions for which trajectories will never exit K irrespective of input variations is called the invariance kernel. This thesis develops theory to characterize the boundary of the invariance kernel and develops an algorithm to compute the exact boundary of the invariance kernel. The algorithm is applied to two simplified wind turbine systems that tap kinetic energy of the turbine to support the frequency of the grid. One system provides power smoothing, and the other provides inertial response. For these models, limits on speed and torque specify a desired region of operation K in the state space, while the wind is represented as a bounded input. The theory developed in the thesis makes it possible to define a measure called the wind disturbance margin. This measure quantifies the largest range of wind variations under which the specified type of grid support may be provided. The wind disturbance margin quantifies how the exploitation of kinetic energy reduces a turbine's tolerance to wind disturbances. The improvement in power smoothing and inertial response made available by the increased speed range of a full converter-interfaced turbine is quantified as an example.

iv Performance prediction of HAWTs is important because it gives an idea about the power production of a HAWT in out of design conditions without making any experiments. Since the experiments of fluid mechanics are difficult to afford, developing some models is very beneficial. There are some models developed about this subject using miscellaneous methods. In this study, one can find &ldquo / Vortex Theory&rdquo / among one of these theories. Some basic 3D aerodynamics was discussed in order to make the reader to understand the main subject of this study. Just after that, performance prediction of constant speed, stall controlled HAWTs was discussed. In order to understand the closeness of this theory to experiments, as a sample, NREL &ldquo / Combined Experiment Rotor&rdquo / was considered. Performances obtained by AEROPOWER, written in Visual Basic 6.0 and Excel combination, and experimental results were compared for different wind velocities. Acceptable results were obtained for wind speeds not much different than the design wind speed. For relatively lower wind speeds, due to &ldquo / turbulence&rdquo / , and for relatively higher wind speeds, due to &ldquo / stall&rdquo / , the program did not give good results. In the first case it has not given any numerical result. Power curves were obtained by only changing the settling angle, and only changing the rotor angular speed using AEROPOWER. It was seen that, both settling angle and rotor rpm values influence the turbine power output significantly.

Designing horizontal-axis wind turbine (HAWT) blades to achieve satisfactory levels of performance starts with knowledge of the aerodynamic forces acting on the blades. In this thesis, HAWT blade design is studied from the aspect of aerodynamic view and the basic principles of the aerodynamic behaviors of HAWTs are investigated. Blade-element momentum theory (BEM) known as also strip theory, which is the current mainstay of aerodynamic design and analysis of HAWT blades, is used for HAWT blade design in this thesis. Firstly, blade design procedure for an optimum rotor according to BEM theory is performed. Then designed blade shape is modified such that modified blade will be lightly loaded regarding the highly loaded of the designed blade and power prediction of modified blade is analyzed. When the designed blade shape is modified, it is seen that the power extracted from the wind is reduced about 10% and the length of modified blade is increased about 5% for the same required power. BLADESIGN which is a user-interface computer program for HAWT blade design is written. It gives blade geometry parameters (chord-length and twist distributions) and design conditions (design tip-speed ratio, design power coefficient and rotor diameter) for the following inputs / power required from a turbine, number of blades, design wind velocity and blade profile type (airfoil type). The program can be used by anyone who may not be intimately concerned with the concepts of blade design procedure and the results taken from the program can be used for further studies.

The increased interest in the offshore wind resource in both industry and academic and the extension of the wind field where offshore wind turbine can be deployed has stimulated quite a number of offshore wind turbines concepts. This thesis presents a design of mono-column platform supported for 5 MW baseline wind turbine developed by the National Renewable Energy Laboratory (NREL), with a single tether anchored to the seabed. The design, based on the pioneer concept SWAY, results from parametric optimized design processes which account for important design considerations in the static and dynamic view, such as the stability, natural frequency, performance requirements as well as the economic feasibility. Fully coupled aero-hydro-servo-elastic model is established in the time-domain simulation tool FAST (Fatigue, Aerodynamics, Structures, and Turbulence) with the hydrodynamic coefficients from HydroGen, an indoor program providing same outputs as the commercial software WAMIT. The optimized model is verified by imitating the frequency-domain approach in FAST and thus comparing the results with the frequency-domain calculations. A number of simulations with various wind and wave conditions are run to explore the effect of wind speed and wave significant height in various water depths. By modifying the optimized model to a downwind turbine with the nacelle rigidly mounted on the tower and the single tether connected to the platform by a subsea swivel, the modified models are more closed to the original SWAY-concept wind turbine. These models are compared based on the platform motion, tether tension, displacement, nacelle velocity and acceleration, resonant behavior as well as the damping of the coupled systems. The results of these comparisons prove the advantage of the modified model in performance. The modified model has also clarified itself a good candidate for deep water deployment.

La forte et rapide croissance de l’énergie éolienne à travers le monde a nécessité la mise en vigueur de nouveaux critères normalisés permettant de l’intégrer dans les réseaux électriques sans affecter la qualité et la stabilité du système, et qui peuvent demander aux éoliennes de participer au réglage de la fréquence dans les réseaux en cas de besoin. Ainsi, les travaux présentés dans cette thèse visent à proposer une solution de loi de commande qui permette aux éoliennes de participer au réglage de la fréquence du réseau. En analysant les limites des correcteurs classiques de types P, PI et PID, nous avons opté pour la commande LQ munie d’une approche multimodèle et qui a montré de bonnes performances aux résultats de simulation. Certaines améliorations ont été ajoutées à cette loi de commande du genre modèle de référence, action intégrale, … afin de permettre une poursuite de puissance autour d’une référence donnée qui change selon la fréquence du réseau / The fast and big growth of wind power around the world required the implementation of new standardized criteria to integrate this kind of energy into electric networks without affecting the quality and stability of the system. These criteria could ask the wind turbines to participate in the network frequency control when necessary. Thus, the works presented in this thesis aim to provide a control law solution that allows wind turbines to participate the grid frequency control. By analyzing the limits of traditional regulators such as P, PI and PID, we opted for the LQ controller combined to a multi-model approach because of the good performances shown in the simulation results. Some improvements were added to this control law: reference model, integral action, ... to allow a power trucking for a given reference that changes with the grid frequency

This thesis approaches the problem of the cost-efficient wind turbine foundation on an onshore site of clayey soil characteristics. The given soil stratigraphy includes a layer of clay and two sands of different density. The characteristics of the soil and the water level that were used as input come from a site in Peloponissos, Greece. The applied wind, static and seismic loads on this study were resolved with the German DIN standards, and other related research and European standards. The safety factors were adjusted for wind turbines. For the pile solution, after the bearing and overturning adequacy against the horizontal and vertical loads was proven with the calculation of the DIN equations, then the model was inserted in the Pfahl program using DIN 4017 equations to calculate settlements. Firstly, a shallow foundation of various dimensions in the clay layer over the water level with all the necessary checks was considered. Afterward, a deep foundation solution of a single bored pile, with reinforcement steel casing, of various diameters was investigated. The different foundation solutions were assessed and compared on a technical and economic basis. As a conclusion, the 0.70 meter diameter single pile was chosen as the best solution because it needs only a few days for construction, and it is the most cost-efficient. The chosen circular footing was of a diameter of 10 meters and 1.5 meter raft thickness, but proved unfeasible because of high excavations costs. The checks on the DIN standards and Eurocode that set the boundaries for the design in the two cases were recognised and possible future work goals were discussed.

The purpose of this research is to propose a novel wind turbine for an omni-flow wind energy system and investigate its aerodynamic performance. The geometry of the novel wind turbine is based upon the impulse turbine technology which has been successful in wave energy. In this study, both numerical and experimental studies were conducted to evaluate the aerodynamic features of this wind turbine. The numerical method was validated by a comparison between numerical and experimental results over a range of tip speed ratios. Results obtained from experiments and simulations indicate that the proposed wind turbine can be employed. Additionally, on the basis of the analyses performed, this new wind turbine has the potential for having a good startup feature, which means that this wind turbine can be suitable for applications in an urban environment. As an important component, the stator of this wind turbine can increase the passing flow velocity by 20%. Meanwhile, the passing flow direction also can be optimised by the stator. Aerodynamics of the wind turbine was analysed under the non-uniform flow condition, because the flow is non-uniform inside the omni-flow wind energy system. It was found that the maximum power coefficient of such a turbine under the non-uniform flow condition is lower than that under the uniform flow condition. Due to the non-uniform flow, the blades experience different flow velocities, and as a consequence, undergo different aerodynamic loads during one operation cycle. Thus the generated torque and thrust on a blade are subjected to frequent and periodical changes. Influences of the geometrical parameters on the aerodynamic characteristics of this wind turbine were investigated. From the initial study, it was found that changes of hub-to-tip ratios, numbers of blades, aerofoils and numbers of guide vanes, can significantly affect the II power performance. Additionally, the wind turbine obtained high values of maximum torque coefficients with changing geometrical parameters.

abstract: An advantage of doubly fed induction generators (DFIGs) as compared to conventional fixed speed wind turbine generators is higher efficiency. This higher efficiency is achieved due to the ability of the DFIG to operate near its optimal turbine efficiency over a wider range of wind speeds through variable speed operation. This is achieved through the application of a back-to-back converter that tightly controls the rotor current and allows for asynchronous operation. In doing so, however, the power electronic converter effectively decouples the inertia of the turbine from the system. Hence, with the increase in penetration of DFIG based wind farms, the effective inertia of the system will be reduced. With this assertion, the present study is aimed at identifying the systematic approach to pinpoint the impact of increased penetration of DFIGs on a large realistic system. The techniques proposed in this work are tested on a large test system representing the Midwestern portion of the U.S. Interconnection. The electromechanical modes that are both detrimentally and beneficially affected by the change in inertia are identified. The combination of small-signal stability analysis coupled with the large disturbance analysis of exciting the mode identified is found to provide a detailed picture of the impact on the system. The work is extended to develop suitable control strategies to mitigate the impact of significant DFIG penetration on a large power system. Supplementary control is developed for the DFIG power converters such that the effective inertia contributed by these wind generators to the system is increased. Results obtained on the large realistic power system indicate that the frequency nadir following a large power impact is effectively improved with the proposed control strategy. The proposed control is also validated against sudden wind speed changes in the form of wind gusts and wind ramps. The beneficial impact in terms of damping power system oscillations is observed, which is validated by eigenvalue analysis. Another control mechanism is developed aiming at designing the power system stabilizer (PSS) for a DFIG similar to the PSS of synchronous machines. Although both the supplementary control strategies serve the purpose of improving the damping of the mode with detrimental impact, better damping performance is observed when the DFIG is equipped with both the controllers. / Dissertation/Thesis / Ph.D. Electrical Engineering 2010

Este trabalho tem como proposta desenvolver um difusor aerodinâmico compacto para uma turbina eólica de pequena escala, objetivando alcançar um melhor start rotacional (menor torque de partida para rotacionar) em baixas velocidades de vento. Um difusor é uma estrutura em forma de aro envolta ao rotor da turbina eólica, sua função é amplificar a captação e aceleração do vento, explorando os efeitos aerodinâmicos das zonas de vórtices de baixa pressão na saída do difusor. O estudo concentrar-se-á na manipulação da geometria dos difusores, analisando como seu design impacta no seu comportamento aerodinâmico impacta na capacidade do difusor equacionar as zonas de alta e baixa pressão ao longo de sua estrutura, essa relação é determinante para o efeito aerodinâmico que acelera o escoamento de ar, resultando em um start rotacional em baixas velocidade de vento. O ponto de partida para este trabalho são os estudos desenvolvidos por Ohya et al. (2010) sobre difusores compactos-flangeados (compact-type brimmed diffuser) para turbinas eólicas, denominado Wind-lens Technology. Para alcançar os objetivos, esta pesquisa vai utilizar simulações por CFD com software de túnel de vento virtual e ensaios experimentais em túnel de vento físico para avaliar o comportamento dinâmico (turbina + difusor). Foram desenvolvidas dezenove geometrias a partir de uma área construtiva padronizada para o design de difusores. Desenvolveu-se também, a partir dos resultados encontrados, um MFI (microseparador de fluxo interno), que consiste em uma estrutura adicional com função de potencializar as zonas de vórtices (baixa pressão) no plano de saída do escoamento de ar dos difusores. Os resultados mostraram que a manipulação da geometria do difusor produziu resultados promissores em comparação com o modelo de referência, alcançando em algumas geometrias de difusores um melhor start rotacional. O MFI mostrou-se eficaz para potencializar as zonas de baixa pressão e melhorou o start rotacional. Ao final, definiu-se dois modelos de difusores e suas respectivas versões com MFI como as melhores opções para o start rotacional. / This work aims to develop a compact wind turbine for a turbine and a small scale, aiming at a better rotational start at low wind speeds (lower starting torque to rotate). A diffuser is a rim-shaped structure wrapped around the wind turbine rotor, its function is to amplify the wind uptake and acceleration, exploiting the aerodynamic effects of the low-pressure vortex zones at the diffuser outlet. The study will focus on the manipulation of the diffuser geometry, analyzing how its design impacts on its aerodynamic behavior, especially on the diffuser's ability to equate the high and low pressure zones along its structure, this relation is decisive for the aerodynamic effect that accelerates the air flow, resulting in a rotational start at low wind speeds. The basis for this work are studies developed by Ohya et al. (2010) on compact-flanged diffusers for wind turbines, called Wind-lens Technology. To achieve the objectives, this research will use CFD simulations with virtual wind tunnel software and experimental tests in physical wind tunnel to evaluate the dynamic behavior (turbine + diffuser). Nineteen geometries were developed from a standardized design area for the design of diffusers. An MFI (internal flow microseparator) has also been developed, which is an additional structure whose function is to potentiate the low pressure zones of the diffusers. The results showed that the manipulation of the diffuser geometry produced promising results in comparison to the reference model, reaching in some conditions superior results in RPM and initial start. The MFI proved to be effective in boosting the low pressure zones and improved the initial start. At the end, two models of diffusers and their respective versions with MFI were defined as the best options for the initial start.

Afin d’optimiser le coût des éoliennes, on choisit souvent d’insérer entre la turbine et la génératrice un multiplicateur de vitesse mécanique à engrenages. Son intérêt est de réduire le coût de la génératrice électrique via la réduction de son couple mécanique. Malheureusement, les multiplicateurs mécaniques sont sujet à des défaillances qui augmentent le coût de fonctionnement de l’éolienne. Ainsi les coûts de maintenance peuvent devenir si importants que certains industriels cherchent à se passer de ce composant, tout particulièrement dans les applications offshore.Parmi les solutions alternatives aux chaînes de conversion à multiplicateur mécanique, une voie innovante consiste à remplacer le multiplicateur à engrenages par une technologie magnétique. Pour évaluer de façon pertinente une telle solution, il est indispensable d'adopter une approche transversale mécatronique considérant à la fois les aspects magnétiques et mécaniques. Une telle approche est originale et pour la mener à bien, il a été nécessaire de développer des modèles multi-physiques afin d’évaluer les performances. Ainsi, les travaux de cette thèse ont porté sur l'élaboration de modèles électromagnétiques, mécaniques et thermiques de deux architectures de multiplicateurs magnétiques. Pour mener à bien une optimisation globale, ces modèles doivent être très performants en termes de compromis temps de calcul / précision. Nous avons enfin réalisé une optimisation globale des parties magnétiques et mécaniques d'un multiplicateur magnétique pour éolienne multi-mégawatt et montré qu'une approche mécatronique permettait d'obtenir de meilleurs résultats qu'une méthode traditionnelle consistant à découpler les dimensionnements magnétiques et mécaniques. / Non communiqué.