As wind power expands rapidly worldwide, it is becoming more common to build wind farms in alpine locations where the wind resources often are good and conflicting interests are few. This is evident in Sweden where a substantial portion of the large wind parks planned are to be built in cold climate locations. The fact that icing of turbine blades and sensors can severely impact the production raises the question how large the losses are. In this thesis 10 wind parks comprising 45 turbines, well dispersed throughout Sweden are investigated. Daily production figures are compared to wind data from the MERRA reanalysis data-set in order to see if it is possible to determine the level of losses during the winter period caused by cold climate. A method is suggested where a relationship between daily production and daily average wind speed is established using representative summer days. This relationship is then used to calculate an expected production for the winter period. Losses are concluded as the difference between expected and actual production. The method did not produce a consistent and reliable result for the sites investigated. However, the method captures the overall trend with higher losses in the north of Sweden compared to the sites in the south where little or no icing is likely. At the sites where icing is expected, losses in the range of 10 to 20% of the annual production were calculated.

[Truncated abstract] The demand for offshore wind turbines is increasing in densely populated areas, such as Europe. These constructions are typically founded on a gravity foundation or a large 'mono pile'. Gravity foundations can only be used at locations where strong soils exist and water depths are limited. Costs associated with a 'mono pile' type foundation contribute to a very large percentage of the total investment costs. This research, therefore, focuses upon a different foundation for offshore wind turbines, namely suction caissons beneath a tripod. This foundation can be used in all kinds of soil types and is cheaper than the 'mono pile' foundation, both in the amount of steel used and installation costs. Cheaper foundations can contribute to a more competitive price for offshore wind energy in comparison with other energy resources. To date, there have been relatively few studies to investigate the behaviour of this type of foundation during the installation process and during operational and ultimate loading for seabed conditions comprising dense sand. Two types of investigations were performed during this research to determine the behaviour of suction caissons beneath a tripod. Firstly, an existing computer program was extended to predict the typical loading conditions for a tripod foundation. Secondly, centrifuge tests on small scale suction caissons were performed to investigate the behaviour during the installation and loading phases. The computer program developed helped to quantify the likely ranges of environmental loading on an offshore wind turbine. For a typical 3 MW wind turbine of 90 m height, the vertical load is low at around 7 MN. During storm conditions the horizontal hydrodynamic load can be in the order of 4 MN. During normal working conditions the horizontal aerodynamic loads can reach 0.4 MN, but can increase to 1.2 MN when the pitch system malfunctions and gusts reach 30 m/s. This aerodynamic load will result in a very large contribution to the overturning moment, due to the high action point of this load. When the wind turbine is placed on top of a tripod, these large moments are counteracted by a push-pull system. ... The development of differential pressure was found to depend on the soil permeability, the extraction speed and a consolidation effect. During cyclic loading no obvious signs of a decrease in resistance were observed. During very fast cyclic loading differential pressures developed, which could increase the drained frictional resistance by approximately 40%. All centrifuge tests results were used to develop methods to predict or back calculate the installation process of suction caissons in sand and layered soil, and the behaviour during tensile and cyclic loading. These methods all use the cone resistance as the main input parameter and predict the force (or required suction) as a function of time, for a given rate of pumping or uplift displacement, in addition to the variation of suction with penetration (or force with uplift displacement). These new methods provide a useful tool in designing a reliable foundation for offshore wind turbines consisting of a tripod arrangement of suction caissons embedded in dense sand.

Η παρούσα διπλωματική έχει ως αντικείμενο την μελέτη και μοντελοποίηση ενός συστήματος μικροστροβίλου και ανεμογεννήτριας ως συστατικά στοιχεία ενός συστήματος κατανεμημένης παραγωγής. Παρουσιάζεται αναλυτικά η έννοια της κατανεμημένης παραγωγής καθώς επίσης και οι τεχνολογίες που χρησιμοποιούνται σε αυτή, ενώ ένα κεφάλαιο είναι αποκλειστικά αφιερωμένο στην τεχνολογία των μικροστρόβιλων. Στο τελευταίο κεφάλαιο γίνεται μια μελέτη της δυναμικής συμπεριφοράς ενός συστήματος μικροστροβίλου σε μεταβλητές συνθήκες φόρτισης καθώς επίσης και της απόκρισης του στην περίπτωση προσθήκης ενός επιπλέον φορτίου, την κάλυψη του οποίου αναλαμβάνει ως εφεδρικό μέσω μία ανεμογεννήτρια σταθερών στροφών. / The current diploma thesis deals with the development of a distributed generation system with a micro-turbine and a wind-turbine as its basic components. What is more, it is given the meaning of the “distributed generation”, we analyze the basic characteristics of it and we present the most important technologies that complete the distributed generation. Furthermore we refer to the micro-turbine technology and its characteristics in the third chapter of this essay. In the last chapter we analyze the dynamic response of a micro-turbine based system in variable load conditions and the response of the system when an additional load, which is powered by a fixed speed wind-turbine, is added in the micro grid.

Η παρούσα διδακτορική εργασία αφορά στο σχεδιασμό και εφαρμογή ελέγχου σε κατανεμημένη παραγωγή ηλεκτρικής ενέργειας ώστε να είναι δυνατή η ένταξή της σε μικροδίκτυα στο δίκτυο χαμηλής τάσης. O σχεδιασμός του ελέγχου αποσκοπεί: 1) Στην δυνατότητα σύνδεσης-αποσύνδεσης (Plug and play) της κατανεμημένης παραγωγής στο μικροδίκτυο χωρίς καμμία ιδιαίτερη παρέμβαση από κεντρικό έλεγχο. 2) Η κατανεμημένη παραγωγή, που περιλαμβάνει συστοιχία κυττάρων καυσίμου και ανεμογεννήτρια, πρέπει να παρέχει στήριξη ενεργού και άεργου ισχύος στο μικροδίκτυο διανομής όταν συμβαίνουν διαταραχές φορτίου. 3) Να τροφοδοτεί όλο το φορτίο σε κατάσταση νησιδοποίησης, στην οποία θα μπορεί να μεταβαίνει είτε λόγω σφάλματος στην μέση τάση είτε γιατί το μικροδίκτυο είναι επιθυμητό να λειτουργεί εσκεμμένα στην προαναφερθείσα κατάσταση. 4) Η μετάβαση από τη διασυνδεδεμένη κατάσταση του μικροδικτύου στη νησιδοποίηση και αντίστροφα να γίνεται με τον ομαλότερο δυνατό τρόπο. Λόγω της έντονης μη γραμμικότητας των υπό μελέτη συστημάτων, στο σχεδιασμό των συστημάτων ελέγχου εφαρμόζεται ασαφής λογική. Τα αποτελέσματα της έρευνας αφορούν τόσο τον θεωρητικό σχεδιασμό των παραπάνω συστημάτων με την βοήθεια προγράμματος εξομοίωσης (Matlab/Simulink) όσο και την πειραματική επιβεβαίωση των συμπερασμάτων στο εργαστήριο. Αρχικά, εξομοιώνεται μια συστοιχία κυττάρων καυσίμου συνδυασμένη με μια μπαταρία (υβριδικό σύστημα) και ενσωματώνεται σε ένα ασθενές δίκτυο διανομής. Ο έλεγχος δοκιμάζεται για τοπικές μεταβολές φορτίου και για μετάβαση από την διασυνδεδεμένη σε νησιδοποιημένη κατάσταση προκειμένου να διαπιστωθούν τα επιθυμητά χαρακτηριστικά του ελέγχου που αναφέρθηκαν. Επιπλέον, γίνεται μια διερεύνηση για το εύρος της αποτελεσματικότητας του ελέγχου για διαφορετικού τύπου γραμμών διανομής. Η συμπεριφορά του ελέγχου κρίνεται ικανοποιητική σε όλες τις περιπτώσεις. Στην συνέχεια, το ίδιο υβριδικό σύστημα (μεγαλύτερης όμως ισχύος) και μια ανεμογεννήτρια με γεννήτρια διπλής τροφοδότησης ενσωματώνονται σε ένα μικροδίκτυο που συνδέεται σε ασθενές δίκτυο διανομής στο πρόγραμμα εξομοίωσης. Ο έλεγχος του υβριδικού συστήματος δεν αλλάζει λόγω της ευελιξίας της ασαφούς λογικής. Ο έλεγχος των μικροπηγών δοκιμάζεται για τοπικές μεταβολές φορτίου και για μετάβαση από την διασυνδεδεμένη σε νησιδοποιημένη κατάσταση και αντιστρόφως για μετάβαση από νησιδοποιημένη σε διασυνδεδεμένη κατάσταση. Επιπλέον, γίνεται μια διερεύνηση της δυναμικής απόκρισης του μικροδικτύου κατά την ύπαρξη μπαταρίας και κατά την απομάκρυνσή της από το μικροδίκτυο. Η συμπεριφορά του ελέγχου κρίνεται ικανοποιητική σε όλες τις περιπτώσεις. Για την πειραματική επιβεβαίωση, αξιοποιείται ένα σύστημα συστοιχίας κυττάρων καυσίμου (Ballard& Nexa System) συνδυασμένο με μια μπαταρία στο χώρο του εργαστηρίου. Εφαρμόζεται η τεχνολογία ψηφιακού ελέγχου σήματος μέσω ενός ψηφιακού επεξεργαστή σήματος (DSP). Έτσι, δίνεται η δυνατότητα σε τμήματα του ελέγχου που έχουν ήδη σχεδιαστεί να μεταφέρονται μέσω ειδικού λογισμικού στον επεξεργαστή και να ενσωματώνονται στην κατανεμημένη παραγωγή με ελάχιστο κόστος. Το πραγματικό σύστημα που δημιουργείται δοκιμάζεται για τοπικές μεταβολές φορτίου. Η συμπεριφορά του ελέγχου κρίνεται ικανοποιητική σε όλες τις περιπτώσεις. Μέσω εκτενών αποτελεσμάτων εξομοίωσης αλλά και πειραματικών, αποδεικνύεται πως ο προτεινόμενος έλεγχος συγκεντρώνει τα επιθυμητά χαρακτηριστικά που αναφέρθηκαν. Αποδεικνύεται, μάλιστα, ιδιαίτερα ευέλικτος και πρακτικός όχι μόνο σε θεωρητικές εφαρμογές αλλά και σε πραγματικά δεδομένα. Το τελευταίο είναι ιδιαίτερα σημαντικό αφού δίνει την δυνατότητα στον μηχανικό με ελάχιστο κόστος και με εύκολο χειρισμό λόγω της ασαφούς λογικής και του ψηφιακού επεξεργαστή να ελέγχει οποιαδήποτε κατανεμημένη παραγωγή (easy engineering) με ελάχιστες διορθωτικές κινήσεις. / Σhe present Ph.d thesis regards to the design and application of the control of the distributed generation (or microsource) so that distributed generation integrates into microgrids at the low voltage side. The design of the control has to achieve the following: 1) Meet the «plug and play» operation mode that implies that a microsource can be added to the microgrid without reengineering the central control and protection of units that are already part of the system. 2) The distributed generation that is a fuel cell system and a wind turbine in this case, have to provide active and reactive power support to the low voltage microgrid in cases of local load disturbances. 3) The distributed generation have to supply the whole demanded power in case of islanding mode of operation either because of a fault at the mean voltage side or because of an intentional disconnection e.g. maintenance work. 4) The transitioning from the interconnected mode to the islanded mode of operation and vice-versa has to be smooth. Due to the intense non linearity of the system, the control design is based on fuzzy logic. The results of this research regard not only to the theoretical design of the above systems via the simulation program Matlab/Simulink but also to the experimental affirmation of the results in the laboratory. Firstly, a fuel cell system combined with a battery bank forming a hybrid system is simulated and integrated into a weak distribution grid. The response of the control system is simulated firstly under a severe step load change under grid connected mode and secondly when a transitioning to islanded operation mode is caused by an upstream supply outage. Also, a study is made in order to ascertain the range of the efficiency of the proposed control for different types of distribution lines. The performance of the control revealed good in all cases. Secondly, the same hybrid system (of bigger nominal power though) and a wind turbine with doubly induction generator are integrated into a microgrid which is connected to a weak distribution grid via the simulation program. The control of the hybrid system remains the same due to the flexibility of fuzzy reasoning. The response of the control system is simulated firstly under a severe step load change under grid connected mode and secondly when a transitioning to islanded operation mode happens and vice-versa when a transitioning from islanded to interconnected operation mode occurs. Also, the transient response of the system is investigated in the cases when the battery bank is part of the microgrid and when it is eliminated. The performance of the control revealed good in all cases. The experimental setup includes a fuel cell system (Ballard& Nexa System) combined with a battery bank. In order to transfer parts of the designed control and integrate them into the experimental setup, the technology of the digital signal control via the digital signal processor (DSP) is exploited. The transfer of the control to the DSP is done through a software program at the minimum costs. The response of the real system is evaluated under a step load. The performance of the control revealed good in all cases.Through a large number of simulation and experimental results, the proposed control proves to meet the desirable requirements that already are mentioned. The proposed controller, also, prove to be particular flexible and practical in real systems. The latter remark makes the designed control suitable in real systems where the engineer tunes the local controller of each distributed generation easier due to fuzzy logic (easy engineering). It has to be reminded, also, that through the digital control technology, the integration of the control by the engineer is at minimum costs.

Η παρούσα διπλωματική εργασία πραγματεύεται τη μελέτη ανεμογεννήτριας με επαγωγική μηχανή διπλής τροφοδοσίας η οποία συνδέεται στο στάτη απευθείας με το δίκτυο και στο δρομέα μέσω ενός back to back μετατροπέα και η συνεχής λειτουργίας της μηχανής σε περιπτώσεις σφαλμάτων στο δίκτυο. Σκοπός της εργασίας είναι αρχικά να μελετηθεί το συνολικό σύστημα της ανεμογεννήτριας θεωρητικά χρησιμοποιώντας το μετασχηματισμό Park για τη μοντελοποίηση του συστήματος στο σύγχρονα στρεφόμενο d-q πλαίσιο αναφοράς. Στη συνέχεια παρατέθηκαν οι εξισώσεις στο χώρο κατάστασης που περιγράφουν το μαθηματικό μοντέλο του συστήματος, το επόμενο βήμα ήταν να περιγραφεί ένα σύστημα ελέγχου το οποίο βασίζεται στους PI ελεγκτές επιλέγοντας τα κατάλληλα κέρδη. Τέλος, προσομοιώνουμε το παραπάνω σύστημα με τη βοήθεια της εφαρμογής Simulink του λογισμικού MATLAB και μελετώντας τα αποτελέσματα, εξάγουμε τα ανάλογα συμπεράσματα. / The present thesis deals with the study of the wind turbine doubly-fed induction generator (DFIG), in which the stator is connected immediately to the grid and the rotor is connected to the grid with a back-to-back converter and the continuous operation of the machine in case of network errors. This thesis will initially consider the overall system of wind turbine theoretically using the Park transformation to model the system in the d-q synchronous rotating reference frame. The following goal was to quote in the state space equations that describe the mathematical model of the system, the next step was to describe a control system based on PI controllers selecting the appropriate profits. Finally, the above system was simulated by using the Simulink of MATLAB and studying the results were exported the appropriate conclusions.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: In this study a new type of gearless, direct-drive wind generator, which can be connected directly to the grid, is proposed. The working characteristics of this generator are based upon the principles of the permanent magnet induction generator (PMIG). By omitting the need for a gearbox and power electronic converter this generator type has several advantages regarding cost and reliability. Although the PMIG is proposed in previous studies as favourable for wind power generation, the mechanical complexity and difficult construction associated with these types of generators, seems to be the main reason why these generator systems are not used. The design methods presented in this study are intended to alleviate these constructional issues by proposing the split-PMIG (S-PMIG), where the stator winding and the induction cage-rotor windings are electromagnetically separated. The machine is basically split into two permanent magnet (PM) machines, a grid connected synchronous generator (SG) unit and a turbine connected induction generator (IG) unit. These two units are mechanically linked by a common PM-rotor. To evaluate this concept a finite element (FE) design analysis is done independently for both machine components. The emphasis of the design optimisation is the minimisation of the cogging torque, while still having a decent performing, easily constructible generator. This generator should also have low load ripple content. Cogging torque can result in the failure of the turbine to start up, especially at low wind speeds. As this is a directly grid connected generator, torque ripple transferred to the common PM-rotor can destabilise the generator. Based upon the FE designs a preliminary S-PMIG system is constructed and practically evaluated. Due to the modular nature of the design it is possible to implement the synchronous generator part of the S-PMIG in conjunction with a grid-connected solid state converter (SSC). This allows for a useful comparison with the S-PMIG system. Promising results are obtained from initial tests of the S-PMIG directly connected to the grid. It is shown that the operation of this generator is stable under a wide range of wind load conditions. However, some important machine design issues are identified from these practical results, which could prove vital in the implementation of future S-PMIG designs. / AFRIKAANSE OPSOMMING: In hierdie studie word 'n nuwe ratkaslose direk-aangedrewe wind generator voorgestel, wat direk aan die krag netwerk gekoppel kan word. Hierdie generator is gebaseer op die beginsels waarop permanente magneet induksie generators (PMIG’s) werk. Deur die ratkas en drywings-elektroniese omsetter uit te laat, het hierdie generator verskeie voordele rakende koste en betroubaarheid. Alhoewel vorige studies hierdie generator voorstel as 'n belowende opsie vir gebruik in wind-energie-stelsels, lyk dit of die meganiese kompleksiteit en moeilike konstruksie die hoof rede is hoekom hierdie generator stelsels tans nie gebruik word nie. Die ontwerpsmetodes wat in hierdie studie voorgestel word poog om die genoemde konstruksie probleme op te los deur die gesplete-PMIG (S-PMIG), waar die stator wikkelings en die induksie masjien kou-rotor wikkelings elektromagneties ontkoppel is, voor te stel. Hierdie masjien is basies verdeel in twee permanente magneet (PM) masjiene, 'n netwerk gekoppelde sinchroongenerator (SG) eenheid en 'n turbine gekoppelde induksie generator (IG) eenheid. Hierdie twee eenhede word gekoppel deur 'n gemeenskaplike PM-rotor. Om hierdie konsep te evalueer word eindige element analise onafhanklik vir beide masjiene gedoen. Die fokus van die ontwerp optimering is die minimalisering van die vertandingsdraaimoment, terwyl die generator steeds goed presteer en maklik is om te vervaardig. Vertandingsdraaimoment kan tot die gevolg hê dat die turbine versuim om te begin draai, veral by lae windsnelhede. Dit is ook belangrik dat die generator 'n lae rimpel inhoud onder las bevat. Omrede hierdie generator direk aan die netwerk gekoppel is, kan draaimoment pulsasies, oorgedra aan die gemeenskaplike PM-rotor, die masjien destabiliseer. Gebaseer op die eindige element ontwerpe, is 'n voorlopige gesplete-PMIG gebou en prakties geëvalueer. As gevolg van die modulêre struktuur van die ontwerp, is dit moontlik om die SG gedeelte van die gesplete- PMIG saam met 'n drywings-elektroniese omsetter te implementeer. Dit laat toe vir 'n nuttige vergelyk tussen die twee stelsels. Belowende eerste toets resultate is verkry van die gesplete-PMIG direk gekoppel aan die netwerk. Daar word gewys dat hierdie generator stabiel oor 'n wye operasionele gebied kan funksioneer. Verskeie belangrike masjien ontwerpsfaktore word ook uitgelig met die praktiese evaluering van die gesplete-PMIG. Hierdie faktore kan uiters belangrik wees in die ontwerp en implementering van toekomstige gesplete-PMIG stelsels. / Sponsored by the Centre for Renewable and Sustainable Energy Studies

Aerogeradores, ou turbinas eólicas, são máquinas instaladas em grandes parques eólicos que convertem a energia cinética do vento em energia elétrica. A definição da separação e da interação entre máquinas é um fator fundamental de análise durante a fase de projeto, pois os chamados efeitos de esteira podem inviabilizar o desenvolvimento de um parque eólico. Em geral, a esteira de um aerogerador está caracterizada por um significativo déficit de velocidade e uma intensificação dos níveis de turbulência, o que ocasiona a diminuição da eficiência aerodinâmica e a redução da vida útil das máquinas localizadas a sotavento. Embora existam diferentes pesquisas destinadas à compreensão e previsão dos efeitos de esteira, o problema permanece como uma questão desafiadora que exige a adoção de ferramentas de alta precisão para sua identificação. Este trabalho apresenta uma metodologia experimental em túnel de vento, para a caracterização e avaliação do campo de escoamento na esteira aerodinâmica de um modelo reduzido, sob diferentes condições de escoamento incidente. Especificamente, investiga-se a influência da turbulência atmosférica para quatro perfis de escoamento: i) uniforme-suave; ii) uniforme-turbulento; iii) lei potencial com expoente α = 0,11; iv) lei potencial com expoente α = 0,23. Todos os casos foram conduzidos sob condições de estratificação neutra, e foi utilizado anemômetro de fio-quente para efetivar as medições dos perfis de velocidade média e intensidade da turbulência, em diferentes posições da esteira. Os resultados mostraram diferenças substanciais no comportamento dos perfis de esteira, em função dos níveis de turbulência incidente. Particularmente, observou-se que o incremento da turbulência atmosférica reduz o déficit de velocidade e promove uma maior mistura turbulenta, o que acelera a dissipação dos efeitos de esteira. Assim, a metodologia experimental em túnel de vento evidencia-se como uma importante ferramenta de análise que possibilita amplo espectro para a investigação, precisão e confiabilidade de projetos eólicos. / Wind turbines are machines installed in large wind farms to convert the wind's kinetic energy into electrical power. For an optimal wind farm siting, it is necessary to take into account the interaction between wind turbine wakes. In general, wake effects are associated with velocity deficit and enhanced turbulence intensity. This may reduce the aerodynamic efficiency and lifetime of downwind turbines, making the project unfeasible. Several experimental and numerical studies have been conducted to unravel the behavior of wind turbine wakes under different inflow conditions. However, current wind farm siting tools are incapable of accurately predicting and assessing its effects. This document presents an experimental methodology in the wind tunnel to survey the influence of the atmospheric turbulence on the wake flow field of a wind turbine model. Specifically, four different flow conditions were investigated: i) uniform-laminar; ii) uniform-turbulent; iii) power law exponent α = 0.11; iv) power law exponent α = 0.23. All cases were developed under neutrally stratified conditions. Hot-wire anemometry was used to obtain high-resolution measurements of the mean velocity and turbulence intensity profiles at different downwind positions. Results show that different turbulence intensity levels of the incoming flow lead to substantial differences in the spatial distribution of the wakes. Particularly, higher ambient turbulence promotes a faster wake recovery and lower velocity deficit. In conclusion, the use of wind tunnel experiments is a trustworthy alternative that brings precision and reliability to wind projects.

Aerogeradores, ou turbinas eólicas, são máquinas instaladas em grandes parques eólicos que convertem a energia cinética do vento em energia elétrica. A definição da separação e da interação entre máquinas é um fator fundamental de análise durante a fase de projeto, pois os chamados efeitos de esteira podem inviabilizar o desenvolvimento de um parque eólico. Em geral, a esteira de um aerogerador está caracterizada por um significativo déficit de velocidade e uma intensificação dos níveis de turbulência, o que ocasiona a diminuição da eficiência aerodinâmica e a redução da vida útil das máquinas localizadas a sotavento. Embora existam diferentes pesquisas destinadas à compreensão e previsão dos efeitos de esteira, o problema permanece como uma questão desafiadora que exige a adoção de ferramentas de alta precisão para sua identificação. Este trabalho apresenta uma metodologia experimental em túnel de vento, para a caracterização e avaliação do campo de escoamento na esteira aerodinâmica de um modelo reduzido, sob diferentes condições de escoamento incidente. Especificamente, investiga-se a influência da turbulência atmosférica para quatro perfis de escoamento: i) uniforme-suave; ii) uniforme-turbulento; iii) lei potencial com expoente α = 0,11; iv) lei potencial com expoente α = 0,23. Todos os casos foram conduzidos sob condições de estratificação neutra, e foi utilizado anemômetro de fio-quente para efetivar as medições dos perfis de velocidade média e intensidade da turbulência, em diferentes posições da esteira. Os resultados mostraram diferenças substanciais no comportamento dos perfis de esteira, em função dos níveis de turbulência incidente. Particularmente, observou-se que o incremento da turbulência atmosférica reduz o déficit de velocidade e promove uma maior mistura turbulenta, o que acelera a dissipação dos efeitos de esteira. Assim, a metodologia experimental em túnel de vento evidencia-se como uma importante ferramenta de análise que possibilita amplo espectro para a investigação, precisão e confiabilidade de projetos eólicos. / Wind turbines are machines installed in large wind farms to convert the wind's kinetic energy into electrical power. For an optimal wind farm siting, it is necessary to take into account the interaction between wind turbine wakes. In general, wake effects are associated with velocity deficit and enhanced turbulence intensity. This may reduce the aerodynamic efficiency and lifetime of downwind turbines, making the project unfeasible. Several experimental and numerical studies have been conducted to unravel the behavior of wind turbine wakes under different inflow conditions. However, current wind farm siting tools are incapable of accurately predicting and assessing its effects. This document presents an experimental methodology in the wind tunnel to survey the influence of the atmospheric turbulence on the wake flow field of a wind turbine model. Specifically, four different flow conditions were investigated: i) uniform-laminar; ii) uniform-turbulent; iii) power law exponent α = 0.11; iv) power law exponent α = 0.23. All cases were developed under neutrally stratified conditions. Hot-wire anemometry was used to obtain high-resolution measurements of the mean velocity and turbulence intensity profiles at different downwind positions. Results show that different turbulence intensity levels of the incoming flow lead to substantial differences in the spatial distribution of the wakes. Particularly, higher ambient turbulence promotes a faster wake recovery and lower velocity deficit. In conclusion, the use of wind tunnel experiments is a trustworthy alternative that brings precision and reliability to wind projects.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / This dissertation addresses the main problems related to doubly-fed induction generators (DFIG) connection to grids with unbalanced voltages. Series compensation is used to achieve balanced voltage on the stator of the machine, avoiding torque oscillations and high currents due to the grid voltage unbalance. The DC-link voltage control proposed reduces the complexity of the converter when compared to the alternatives considered in the literature. The close to no-load operation of the DFIG is also depicted. Simulation results for a commercial large scale generator are presented to demonstrate the performance of the system with the proposed controller. / Esta dissertação trata dos principais problemas relacionados à conexão de geradores duplamente alimentados à rede elétrica com tensões desequilibradas. Compensação série é usada para equilibrar as tensões no estator da máquina, evitando oscilações de conjugado e altas correntes de seqüência negativa devido aos desequilíbrios das tensões no ponto de conexão do gerador. O controle da tensão do barramento CC é proposto tal que é possível reduzir a complexidade do conversor quando comparado com outras alternativas apresentadas na literatura. A operação próximo à carga nula é apresentada. Resultados de simulação para um gerador em escala comercial são apresentados para demonstrar o desempenho do sistema proposto.

Thesis submitted in fulfilment of the requirements for the degree MAGISTER TECHNOLOGIAE: Mechanical Engineering in the FACULTY OF ENGINEERING at the CAPE PENINSULA UNIVERSITY OF TECHNOLOGY, 2012 / The amount of power a horizontal axis wind turbine (HAWT) can produce is determined by two main factors, wind velocity and rotor swept area. Theory dictates that the power production of a horizontal wind turbine is related to the cube of wind velocity and the square of the turbine diameter (or radius). The power produced at any given time is thus dependent on of the wind velocity and the rotor swept area of the turbine. Wind is variable in availability and consistency. Very little can be done to effect the wind velocity passing through the turbine rotor area and its effect is minimal. Thus understandably if more power is required, from the same wind velocity, the rotor diameter must be increased. A variable length blade can adapt lengthwise to accommodate low wind velocities and similarly high wind velocities during extreme conditions, thus increasing the operational time and power production of the turbine. The work undertaken in this thesis is a comparative study between standard design, fixed length blades to that of a modified design, variable length blade. The project entailed the design and development of small diameter HAWT blades and experimental testing. The turbine blades were designed using applicable theory and manufactured from available materials. For the experiments, the turbine was mounted on a vehicle and driven at various speeds. Due to size limitations, no dynamic adaption was done during testing. The variable length design blade was obtained by cutting increments off. The results obtained from each test were compared at corresponding points and conditions. Final interpretation of results lead to the conclusion that by increasing or decreasing the turbine blade length the area of turbine energy capture can be adjusted to affect the amount of power produced. Additional benefits included, force reduction during extreme operating conditions, extended production period for the turbine and a mechanical start up method during low wind speeds. The financial feasibility did not form part of the scope of this thesis and the technical feasibility of the concept can be thoroughly addressed in future research.