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published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

This thesis presents and examines the concept that the output of a wound-rotor induction generator (WRIG) can be limited by means of linking to external impedances for wind-turbine generating system application. An 80kW-WRIG is simulated as a model to examine the control of the output power vs. speed characteristic. Model of WRIG derived from per phase equivalent circuit is organized, then it is estimated how much external impedances affect the characteristic of output power for it to approach to a typical wind-turbine curve. Practical tests are performed using 80kW-WRIG in testing lab to validate the simulation data. In addition, a smaller WRIG, connected on same shaft as 80kW-WRIG, is designed to extend the range of wind speed. Also external impedances with smaller WRIG are chosen to extract optimum power from wind-turbine. Finally, passively controlled tandem WRIGs are shown to have the capability to optimize wind-turbine energy extraction when controlled entirely by external impedances. / Graduation date: 2003

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Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The aim of this project is to study the structural design and costing of various designs of tall wind turbine towers and the associated foundations in a South African context. Speci c design guidelines are proposed for the design of tubular steel, concrete and concrete-steel hybrid towers and foundations for hub heights of 80, 100 and 120m. Additionally, a conclusion will be made as to whether the concrete and hybrid towers are a viable alternative to steel towers at higher hub heights. To accomplish this, three of each type of tower (steel concrete and hybrid) and their foundations were designed according to the relevant design standards. The designs were then veri ed using the Abaqus nite element software. The costs of the designs for a South African environment were subsequently calculated according to the increases in material cost, as a function of the increase in hub height. It was found that for the chosen design assumptions, the foundations for the concrete and hybrid towers are less material intensive, and therefore cheaper, than the steel towers. The material costs of the concrete and hybrid towers were also shown to be lower than the material costs of the steel towers, especially at hub heights of 100 to 120m. For the circumstances in this project, it was found that an increase in hub height causes an increase in energy generation of 3.52 and 6.28 percent for 80m to 100m and 80m to 120m hub heights, respectively. It is therefore deduced that, given the trends in the design and cost associated with increasing hub heights, the concrete and hybrid towers become viable alternatives to the conventional steel towers at hub heights of 100 to 120m in height. / AFRIKAANSE OPSOMMING: Die doel van hierdie projek is om die strukturele ontwerp en koste van verskillende soorte ho e wind turbines en die toepaslike fondamente vir 'n Suid-Afrikaanse konteks te bestudeer. Spesi eke riglyne word voorgestel vir die ontwerp van silindervormige staal, beton en beton-staal hibriede torings en fondamente vir naafhoogte van 80, 100 en 120m. 'n Gevolgtrekking oor die lewensvatbaarheid van die beton en hibriede torings, in vergelyking met die gewone staal torings teen naafhoogte van 100 tot 120m sal ook gemaak word. Met die bogenoemde as doel, is drie van elke tipe toring (staal, beton en hibried) en hul fondamente volgens die toepaslike standaarde ontwerp. Daarna is die integriteit van die ontwerpe getoets en bevestig deur gebruik van die Abaqus eindige-element-metode sagteware. Ten slotte, die kostes van die ontwerpe vir 'n Suid Afrikaanse omgewing is bereken en die verandering in materiaalkoste uitgedruk as 'n funksie van die verhoging in naafhoogte. Daar is gevind dat, vir die aannames in die ontwerp, die fondamente van die beton en hybrid torings minder materiaal benodig, en dus goedkoper is as die staal torings. Verder, is die materiaalkoste van die beton en hibriede torings laer as die van die staal torings, veral vir naafhoogtes van 100 tot 120m. Verder, is daar vir die omstandighede in hierdie projek gevind dat ho er naafhoogtes stygings in energie-opwekking van 3.52 persent vir naafhoogte stygings van 80m tot 100m, en stygings van 6.28 persent vir naafhoogte stygings van 80m tot 120m lewer. Daar word dus tot die gevolgtrekking gekom dat, gegewe die tendense in die ontwerp en materiaal koste wat verband hou met die verhoging van die naafhoogte, die beton en hibriede torings 'n lewensvatbare alternatief vir die konvensionele staal torings vir naafhoogtes van 100 tot 120m word.

Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Exponential growth of the global wind turbine market has led to a significant increase in the capacity of wind turbine generators. Modern turbines require higher support structures as higher wind speeds combined with longer blades are necessary to increase their generating capacity. The standard 80-90 m tower is thus not economically viable anymore. Transportation logistics of large steel towers has led to concrete towers becoming a viable option. There are currently no design codes dealing exclusively with the design of concrete wind turbine towers. The aim of this project is to investigate and highlight important aspects of the design process of a normally reinforced high strength concrete wind turbine tower. The tower was designed using nonlinear finite element modelling as a design tool to accurately design the tower for various loads and load cases. An analytical design method was developed that can be used in the preliminary design stage. Finally, the importance of the soil-structure interaction was investigated through a sensitivity analysis. It was found that the formation of cracks greatly affected the stiffness of the structure and that the reduction in stiffness increased the deflection significantly. It was also found that a structure that has sufficient strength to resist the ULS loads may not necessarily comply with the maximum deflection limit for the SLS. The concrete strength class required was not only determined by the maximum compression stress the concrete would experience, but also by the stiffness required to ensure that the tower frequency is within the turbine’s working frequency. The dynamic behaviour of the tower was also affected by the formation of cracks. The fundamental frequency of the tower was reduced by 46% after the SLS loads were applied. It was found that the soil preparation for the foundation plays a vital role in ensuring that the tower frequency is not reduced to a level where it falls outside the turbine working frequency. / AFRIKAANSE OPSOMMING: Die eksponensiële groei van die globale wind turbine mark het gelei tot ʼn beduidende toename in die opwekkingskapasiteit van wind turbine kragopwekkers. Moderne turbines benodig hoër ondersteuningstrukture om hulle opwekkingskapasiteit te verhoog en daarom is die standaard 80-90 m toring nie meer geskik nie. Die vervoer logistiek van groot staal torings het daartoe gelei dat beton torings ʼn lewensvatbare opsie geword het. Daar is huidiglik geen ontwerpkodes wat uitsluitlik handel met die ontwerp van beton wind turbine torings nie. Die doel van hierdie projek is om die ontwerp proses van ʼn bewapende hoë sterkte beton wind turbine toring te ondersoek en belangrike aspekte uit te lig. Die toring word ontwerp deur ʼn nie-liniêre eindige element model te gebruik as ʼn ontwerp hulpmiddel, om die toring akkuraat te ontwerp vir verskeie laste en lasgevalle. ʼn Analitiese ontwerpmetode is ontwikkel wat gebruik kan word in die voorlopige ontwerpfase. Laastens is die grond-struktuur interaksie ondersoek deur ʼn sensitiwiteitsanalise. Daar is gevind dat die vorming van krake die styfheid van die struktuur aansienlik beïnvloed en dat die vermindering in styfheid die defleksie beduidend vermeerder. Daar is ook gevind dat ʼn struktuur wat voldoende sterkte het om die uiterste lastoestande te weerstaan, nie noodwendig voldoen aan die maksimum defleksiegrens vir die diens lastoestande nie. Die beton sterkte klas wat benodig is, is nie net bepaal deur die maksimum druk spanning wat die beton sal ondervind nie, maar ook deur die styfheid wat vereis word om te verseker dat die toring se frekwensie binne die turbine se werksfrekwensie val. Die dinamiese gedrag van die toring is ook beïnvloed deur die vorming van krake. Die fundamentele frekwensie van die toring is verlaag met 46% nadat die diens lastoestande toegepas is. Daar is gevind dat die grond voorbereiding vir die fondasie ʼn belangrike rol speel om te verseker dat die toring se frekwensie nie verlaag word tot ʼn vlak waar dit buite die turbine se werksfrekwensie val nie.

The goal of this Thesis is to demonstrate, through experimentation, that ocean waves have a positive effect on the performance of an offshore wind turbine. A scale model wind turbine was placed into a wave tank that was completely covered and fitted with a variable speed fan to create different wind and wave conditions for testing. Through testing, different power coefficient vs. tip speed ratio graphs were created and a change in power coefficient was observed between steady operating conditions and operating conditions with waves. The results show a promising increase in power production for offshore wind turbines when allowed to operate with the induced motion caused by the amplitude and frequency of water waves created. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2013.

Wind energy is a relatively young but rapidly expanding industry. In order for wind generation to be cost effective, it must produce energy at a minimum cost per dollar of investment. Performance characteristics such as power output versus wind speed must be optimized in order to compete with other energy sources. Also, if the utility uses wind power for a part of its generation, the output power of these turbines must have the same high quality and reliability when it enters the utility grid. The ability to vary operating speed is important in wind generation because it allows for an optimization of the transfer of power from the wind to the turbine shaft. Doubly-fed induction generators (DFIG) are an interesting solution for variable-speed systems with limited variable speed range, and are typically used in wind energy conversion systems. / The objectives of this thesis are to implement a decoupled P-Q control of a DFIG that does not rely on mechanical sensors and to design a speed and position sensorless algorithm that is robust to variations of the values of the machine parameters. The sensorless DFIG control algorithm presented in this thesis is based on a modified phasedlock loop with an improved positioning algorithm. With the measured stator voltages, stator and rotor currents, the speed and position of the DFIG are estimated. The speed is estimated independent of machine parameters, which results in a significant improvement in speed control robustness to parameter variations. In addition, the algorithm avoids using differentiation, which significantly improves its immunity to noise and does not require the measurement of the rotor voltages. Also, it is shown that the positioning algorithm has an improved operation in generator mode. Although the accuracy of the positioning algorithm is depended on machine parameters; it is shown with the designed controller that the P-Q decoupled control is robust to changes of machine parameters. Theoretical and simulation results are validated on an experimental setup.

The typical methodology for analytically designing a wind turbine blade is by means of blade element momentum (BEM) theory, whereby the aerofoil angle of attack is optimized to achieve a maximum lift-to-drag ratio. This research aims to show that an alternative optimisation methodology could yield better results, especially in gusty and turbulent wind conditions. This alternative method looks at increasing the aerofoil Reynolds number by increasing the aerofoil chord length. The increased Reynolds number generally increases the e_ectiveness of the aerofoil which would result in a higher or similar lift-to-drag ratio (even at the decreased angle of attacked require to maintain the turbine thrust coe_cient). The bene_t of this design is a atter power curve which causes the turbine to be less sensitive to uctuating winds. Also, the turbine has more torque at startup, allowing for operatation in lower wind speeds. This research is assumed to only be applicable to small wind turbines which operated in a low Reynolds number regime (<500 000), where Reynolds number manipulation is most advantageous.

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