Thesis (MScEng)--Stellenbosch University, 2012 / ENGLISH ABSTRACT: Wind energy is a very current topic, both locally and internationally. It is one of the most rapidly growing renewable energy sources with installed capacity doubling every three years. South Africa's installed wind energy currently accounts for only 10 MW of the 197 GW worldwide installed capacity. With a 10 TWh renewable energy production target set for 2013 by the South African government, renewable energy projects have gained momentum in recent years. This target, together with data from case studies and reports on resource planning and technical requirements, shows that South Africa is well positioned for the implementation of wind energy sources. All this development in the local wind generation market creates a need for local knowledge in the field of wind energy as well as a need to efficiently model and analyse wind turbine systems and grid interactions for local operating conditions. Although the relevant model topologies are well established, obtaining or deriving appropriate parameter values from first principles remains problematic. Some parameters are also dependent on operating conditions and are best determined from site measurements using parameter estimation methodologies. One of the objectives of this project is to investigate whether the system parameter values can be obtained by performing parameter estimation on the model of a wind turbine system. The models used for parameter estimation processes require fast simulation times. Therefore, basic C-code S-function models of the wind turbine system components, i.e., the wind turbine blade, gearbox and generator, were developed and compiled as a Simulink library. These library components were then used for the parameter estimation process. The developed models, as well as the complete wind turbine system model, were validated and their performance evaluated, by comparing them to existing Simulink block models. These models all proved to be accurate and all showed reductions in simulation times. The principle of performing parameter estimation on C-code S-function models is proven by case studies performed on the individual models and the complete wind turbine system. The power coefficient matrix parameter values of the individual turbine blade model estimated with 100% accuracy for the excited elements. The individual gearbox parameter values all estimated accurately with errors below 2.5%. The parameter values of the individual generator models were estimated accurately for the ABC model, with errors below 4%, and less accurately for the DQ model with errors below 13%. The estimation results obtained for the complete wind turbine system model showed that the parameter values of the gearbox model and generator model were estimated accurately when the system model was excited through a step in angular velocity and steps in amplitude of the stator voltages respectively. A final estimation showed that a combination of gearbox and generator parameter values were accurately estimated when the model was excited through both a step in angular velocity and steps in the amplitude of the stator voltages. / AFRIKAANSE OPSOMMING: Windenergie is 'n baie aktuele onderwerp beide plaaslik en internasionaal. Windenergie is een van die vinnigste groeiende hernubare energie bronne met die geïnstalleerde kapasiteit wat driejaarliks verdubbel. Suid-Afrika se geïnstalleerde windenergie maak tans slegs 10 MW uit van die wêreldwye geïnstalleerde kapasiteit van 197 GW. Die Suid-Afrikaanse regering het ’n 10 TWh hernubare-energie produksie teiken gestel vir 2013. As gevolg hiervan het hernubare-energie projekte die laaste paar jaar momentum gekry. Hierdie teiken, tesame met die data van gevallestudies en verslae oor hulpbronbeplanning en tegniese vereistes, toon dat Suid-Afrika goed geposisioneer is vir die implementering van windenergiebronne. Hierdie ontwikkelinge in die plaaslike windenergie mark skep ’n behoefte aan plaaslike kennis op die gebied van windenergie, asook die behoefte vir ’n doeltreffende wyse vir die modellering en analisering van windturbine stelsels en netwerk integrasie vir plaaslike werkskondisies. Alhoewel die betrokke model topologieë reeds goed gevestig is, is die verkryging van toepaslike parameter waardes vanuit eerste beginsels steeds problematies. Sommige parameters is ook afhanklik van die werkskondisies en kan die beste bepaal word deur gebruik te maak van parameter estimasie metodologieë vanaf terrein metings. Een van die doelwitte van die projek is om ondersoek in te stel na die moontlikheid om die stelsel parameter waardes te verkry deur parameter estimasie toe te pas op ’n windturbine stelsel. Die modelle wat gebruik word vir die parameter estimasie prosesse benodig vinnige simulasie tye. Daarom is basiese C-kode S-funksie modelle vir die komponente van windturbine stelsels, d.w.s., die wind turbine lemme, ratkas en generator, ontwikkel en saamgestel as ’n Simulink biblioteek. Die komponente in hierdie biblioteek was toe gebruik vir die parameter estimasie proses. Die ontwikkelde modelle sowel as die hele windturbine stelsel model was gevalideer en hul werksverrigting geëvalueer, deur dit te vergelyk met bestaande Simulink blok modelle. Hierdie modelle het almal getoon dat hulle akkuraat is en het almal ’n vermindering in simulasie tyd getoon. Die beginsel van parameter estimasie wat uitgevoer word op C-kode S-funksie modelle, is bewys deur gevallestudies wat op die individuele modelle en die hele windturbine stelsel model uitgevoer was. Die geperturbeerde elemente van die kragkoëffisiënt-matriks arameter van die individuele turbine lemme model se waardes het 100% akkuraatheid geëstimeer. Die individuele ratkas model se parameter waardes was almal akkuraat geëstimeer, met foute kleiner as 2.5%. Die individuele generator modelle se parameter waardes was akkuraat geëstimeer vir die ABC model, met foute kleiner as 4%, en minder akkuraat vir die DQ model, met foute kleiner as 13%. Die resultate wat verkry is van die estimasie wat uitgevoer is op die volledige windturbine stelsel model, het getoon dat die parameter waardes van die ratkas model en die generator model akkuraat geëstimeer word, wanneer die stelsel model onderskeidelik deur ’n trap in die hoeksnelheid en trappe in die amplitude van die stator spannings geperturbeer word. ’n Finale estimasie het getoon dat ’n kombinasie van ratkas en generator parameter waardes akkuraat geëstimeer kan word as die model deur beide die trap in hoeksnelheid en die trappe in die amplitude van die stator spannings geperturbeer word.

Efeitos termoelétricos descrevem o surgimento de campos elétricos em função de gradientes de temperatura e vice-versa. Neste trabalho investigamos as propriedades termoelétricas de materiais de baixa dimensionalidade e nanoestruturas através de cálculos de primeiros princípios das propriedades de transporte destes sistemas, usando o código TRANSAMPA, que é baseado em funções de Green fora do equilíbrio e do código SIESTA, baseado em teoria do funcional da densidade. Inicialmente estudamos nanofitas de grafeno e como estas são alteradas pela presença de impurezas substitucionais de Boro e Nitrogênio. Entre os principais resultados, mostramos que fitas na configuração ferromagnética apresentam efeito Seebeck dependente do spin, que pode ser ajustado por efeito de campo. A seguir, vemos que o coeficiente Seebeck (S) em bicamadas de grafeno pode ser ajustado por potenciais de gate, de forma a escolher os portadores de carga, atingindo S =_250 _V/K. Também estudamos a dependência de S com a temperatura (T) e o tamanho do gate, calculamos a condutividade térmica por dinâmica molecular e a eficiência termoelétrica (ZT). Na seqüencia, mostramos que grafeno dopado com Mn mostra caloritrônica de spin ajustável via gate e como a termocorrente varia com T e _T. Finalmente, calculamos as propriedades termoelétrica de uma junção molecular Au-BDT-Au e como elas variam em função do alongamento da junção. Também propomos um esquema geral para maximizar ZT de junções moleculares em geral. / Thermoelectric effects describe how electric fields arise in response to temperature gradients and vice versa. In this thesis we investigate the thermoelectric properties of low-dimensional materials and nanostructures theoretically. We perform ab initio calculations of the electronic transport properties using the TRANSAMPA code, based in nonequilibrium Greens functions, and the SIESTA code, based in density functional theory. First, we study graphene nanoribbons and how their properties are altered by substitutional impurities. Among our main results for this system, we show that ribbons in the ferromagnetic configuration present spin-dependent Seebeck effect, which can be tuned by a field effect. We show that the Seebeck coefficient (S) of bilayer graphene is highly tunable by a gate potential, with ambipolar behavior, reaching S = _250 _V/K. We also study how S varies with temperature (T) and gate length. We calculate its thermal conductivity by molecular dynamics, and its thermoelectric efficiency (ZT ). Then, we show that Mn doped graphene features a gate-tunable spin-dependent S, which is robust under changes in T and _T, rendering this material suitable for spin caloritronics. Finally, we calculate how the thermoelectric properties of an Au-BDT-Au molecular junction vary with mechanical stretching, and propose a general recipe to improve ZT in molecular junctions in general.

AC/DC Buck converters have been used widely in many applications from cell phones to vehicle battery chargers. Due to their importance many researchers have been studying their behavior to improve their efficiency and reduce their size and/or cost. One of the most common defects of these converters, whether they are used for high power or low power applications, is the unwanted ripples in the input voltage across the input stage. It is believed that these ripples are caused by the interaction between the converter itself or its controller with the rectifier required to change the AC input into DC followed by an input filter. Many strategies have been suggested to tackle this problem. A new strategy to improve the controller of that converter was suggested by M. Lenells [1] and it was based on a Hamiltonian model for the 3-phase AC/DC converter together with its rectifier. As a first step, we simulated this model for a single-phase DC/DC buck converter only using the so-called S-Functions in MATLAB/SIMULINK. Then we could find a control law that would reduce the ripples in the input voltage and keep the output voltage constant simultaneously. In this report, we present this model and its simulation to pave the way for the control and simulation of the 3-phase AC/DC converter.

Efeitos termoelétricos descrevem o surgimento de campos elétricos em função de gradientes de temperatura e vice-versa. Neste trabalho investigamos as propriedades termoelétricas de materiais de baixa dimensionalidade e nanoestruturas através de cálculos de primeiros princípios das propriedades de transporte destes sistemas, usando o código TRANSAMPA, que é baseado em funções de Green fora do equilíbrio e do código SIESTA, baseado em teoria do funcional da densidade. Inicialmente estudamos nanofitas de grafeno e como estas são alteradas pela presença de impurezas substitucionais de Boro e Nitrogênio. Entre os principais resultados, mostramos que fitas na configuração ferromagnética apresentam efeito Seebeck dependente do spin, que pode ser ajustado por efeito de campo. A seguir, vemos que o coeficiente Seebeck (S) em bicamadas de grafeno pode ser ajustado por potenciais de gate, de forma a escolher os portadores de carga, atingindo S =_250 _V/K. Também estudamos a dependência de S com a temperatura (T) e o tamanho do gate, calculamos a condutividade térmica por dinâmica molecular e a eficiência termoelétrica (ZT). Na seqüencia, mostramos que grafeno dopado com Mn mostra caloritrônica de spin ajustável via gate e como a termocorrente varia com T e _T. Finalmente, calculamos as propriedades termoelétrica de uma junção molecular Au-BDT-Au e como elas variam em função do alongamento da junção. Também propomos um esquema geral para maximizar ZT de junções moleculares em geral. / Thermoelectric effects describe how electric fields arise in response to temperature gradients and vice versa. In this thesis we investigate the thermoelectric properties of low-dimensional materials and nanostructures theoretically. We perform ab initio calculations of the electronic transport properties using the TRANSAMPA code, based in nonequilibrium Greens functions, and the SIESTA code, based in density functional theory. First, we study graphene nanoribbons and how their properties are altered by substitutional impurities. Among our main results for this system, we show that ribbons in the ferromagnetic configuration present spin-dependent Seebeck effect, which can be tuned by a field effect. We show that the Seebeck coefficient (S) of bilayer graphene is highly tunable by a gate potential, with ambipolar behavior, reaching S = _250 _V/K. We also study how S varies with temperature (T) and gate length. We calculate its thermal conductivity by molecular dynamics, and its thermoelectric efficiency (ZT ). Then, we show that Mn doped graphene features a gate-tunable spin-dependent S, which is robust under changes in T and _T, rendering this material suitable for spin caloritronics. Finally, we calculate how the thermoelectric properties of an Au-BDT-Au molecular junction vary with mechanical stretching, and propose a general recipe to improve ZT in molecular junctions in general.

A hierarquia de equações de Boltzmann que descreve a temperatura e polarização da radiação cósmica de fundo ´e tratada no espaço das posições. Mostramos neste formalismo que temperatura e polarização podem ser descritas como medias dos termos de fonte ponderados por probabilidades associadas a um problema de voos aleatórios. Decorre da estrutura geral da hierarquia que se pode fazer uma expansão da temperatura e polarização em termos do numero de espalhamentos ocorridos durante a recombinação. Incorporamos o efeito de lentes gravitacionais sobre a radiação cósmica de fundo tirando proveito da estrutura das equações no espaço das posições. Mostraremos que o efeito ´e incorporado através de correções aos coeficientes da decomposição angular dos campos de temperatura e polarização. Para descrever o efeito de lentes gravitacionais fazemos uma revisão sobre resultados formais da teoria e apresentamos uma derivação de equações centrais em espaços-tempo arbitrários. / The Boltzmann hierarchy describing the temperature and polarization of the cosmic microwave background is presented in a position space formalism. We show that temperature and polarization can be described in terms of averages over source terms weighted by probabilities densities that appear in problems of random flights. The temperature and polarization signals can be expanded in terms of the number of scatterings photons suffered during the recombination. The gravitational lensing effects are incorporated over the free-propagation phase of the CMB photons. This effect can be included in the position space formalism as a correction to the expansion coefficients of the temperature and polarization fields. The bases of the theory of gravitational lensing are also presented and a rederivation of the central equations of the theory in arbitrary spacetimes is here developed.

Full wave analysis of microstrip lines at microwave frequencies is performed by using method of moments in conjunction with closed-form spatial domain Green&rsquo / s functions. The Green&rsquo / s functions are in general Sommerfeld-type integrals which are computationally expensive. To improve the efficiency of the technique, Green&rsquo / s functions are approximated by their closed-forms. Microstrip lines are excited by arbitrarily located current sources and are terminated by complex loads at both ends. Current distributions over microstrip lines are represented by rooftop basis functions. At first step, the current distribution over a single microstrip line is calculated. Next, the calculation of the current distributions over coupled microstrip lines is performed. The technique is then, applied to directional couplers. Using the current distributions obtained by the analysis, the scattering parameters of the structures are evaluated by using Prony&rsquo / s method. The results are compared with the ones gathered by using simulation software tools, CNL/2&trade / and Agilent Advanced Design System&trade / (ADS).

In the first part of the thesis, the 2-D structures in stratified media are analyzed using an efficient MoM technique. The method is used to optimize transmitted or reflected electric fields from the 2-D structures. The genetic algorithm is used in the optimization process. In the second part a 3-D MoM technique is implemented to analyze multilayered structures with periodically implanted material blocks. Using the method, the dispersion and reflection characteristics of the structure are calculated for different configurations. The results are compared with the results found in the literature and it is seen that they are in good agreement. Asymptotic Waveform Evaluation (AWE) technique is utilized to obtain the Pade approximation of the solution in terms of frequency. The high order derivatives that are required by the AWE technique are calculated through Automatic Differentiation technique. Using the AWE method, the dispersion diagram and reflection characteristics of the periodic structures are obtained in a shorter time. The results are compared with the ones obtained through direct calculation and it is seen that they are in perfect agreement. The reflection coefficients that are obtained from the 3-D MoM procedure are used to calculate Green&rsquo / s functions that approximate electric field of an infinitesimal dipole on the periodically implanted substrate. Using the calculated Green&rsquo / s functions and the spectral domain MoM procedure, dispersion characteristics of a microstrip line on the periodically implanted substrate are obtained.

A hierarquia de equações de Boltzmann que descreve a temperatura e polarização da radiação cósmica de fundo ´e tratada no espaço das posições. Mostramos neste formalismo que temperatura e polarização podem ser descritas como medias dos termos de fonte ponderados por probabilidades associadas a um problema de voos aleatórios. Decorre da estrutura geral da hierarquia que se pode fazer uma expansão da temperatura e polarização em termos do numero de espalhamentos ocorridos durante a recombinação. Incorporamos o efeito de lentes gravitacionais sobre a radiação cósmica de fundo tirando proveito da estrutura das equações no espaço das posições. Mostraremos que o efeito ´e incorporado através de correções aos coeficientes da decomposição angular dos campos de temperatura e polarização. Para descrever o efeito de lentes gravitacionais fazemos uma revisão sobre resultados formais da teoria e apresentamos uma derivação de equações centrais em espaços-tempo arbitrários. / The Boltzmann hierarchy describing the temperature and polarization of the cosmic microwave background is presented in a position space formalism. We show that temperature and polarization can be described in terms of averages over source terms weighted by probabilities densities that appear in problems of random flights. The temperature and polarization signals can be expanded in terms of the number of scatterings photons suffered during the recombination. The gravitational lensing effects are incorporated over the free-propagation phase of the CMB photons. This effect can be included in the position space formalism as a correction to the expansion coefficients of the temperature and polarization fields. The bases of the theory of gravitational lensing are also presented and a rederivation of the central equations of the theory in arbitrary spacetimes is here developed.

Method of Moments (MoM)/Green&rsquo / s function formulation is developed for the analysis of electromagnetic radiation from planar rectangular microstrip antennas with different feeding techniques. Investigated structures are microstrip line fed patch antenna, proximity coupled patch antenna and slot coupled patch antenna. For all these structures equivalent problems are defined. Then, integral equations where currents are the unknowns are obtained from boundary conditions and by using spectral domain representation of Green&rsquo / s functions. Finally, MoM is applied to convert these integral equations to a system of linear equations. Currents on the conducting surfaces as well as equivalent magnetic currents on the apertures are modeled as a sum of piecewise sinusoidal subdomain basis functions with unknown coefficients which are calculated by solving the system of linear equations. Based on the formulations provided in this study, a Fortran code is developed. Numerical results calculated by using the code are presented in the form of patch and line currents and input impedances. Presented results are in good agreement with the results given in the literature.

In this thesis, an analysis technique for the slot coupled patch antennas using MoM in conjunction with the closed form Green&rsquo / s functions is presented. Slot coupled patch antennas are fed by a microstrip open stub which is coupled to the patch through an electrically small slot. Current distributions over the microstrip line, slot line and the patch are represented by rooftop basis functions. First, a relatively simple structure, microstrip coupled slot line is investigated using the proposed technique. Then the method is extended to the slot coupled patch antenna geometry. By using the method, current distributions on the feedline and the patch are calculated for a generic slot coupled patch antenna. Then by using the distributions, return scattering parameters of the antenna is approximated with complex exponentials using Prony&rsquo / s method. A parametric study is carried out to observe the effect of each antenna component on the antenna performance. Current distributions and return loss calculations are repeated for modified antennas to observe and demonstrate the performance differences. All simulations are verified using HFSS&reg / software and the results available in the literature.