Modern power systems operate much closer to their stability limits than before. With the introduction of highly sensitive industrial and residential loads, the loss of system stability becomes increasingly costly. Reinforcing the power grid by installing additional transmission lines, creating more complicated meshed networks and increasing the voltage level are among the effective, yet expensive solutions. An alternative approach is to improve the performance of the existing power system components by incorporating more intelligent control techniques. This can be achieved in two ways: introducing intelligent local controllers for the existing components in the power network in order to employ their utmost capabilities, and implementing global intelligent schemes for optimizing the performance of multiple local controllers based on an objective function associated with the overall performance of the power system. Both these aspects are investigated in this thesis. In the first section, artificial neural networks are adopted for designing an optimal nonlinear controller for a static compensator (STATCOM) connected to a multimachine power system. The neurocontroller implementation is based on the adaptive critic designs (ACD) technique and provides an optimal control policy over the infinite horizon time of the problem. The ACD based neurocontroller outperforms a conventional controller both in terms of improving the power system dynamic stability and reducing the control effort required. The second section investigates the further improvement of the power system behavior by introducing an ACD based neurocontroller for hierarchical control of a multimachine power system. The proposed wide area controller improves the power system dynamic stability by generating optimal control signals as auxiliary reference signals for the synchronous generators automatic voltage regulators and the STATCOM line voltage controller. This multilevel hierarchical control scheme forces the different controllers throughout the power system to optimally respond to any fault or disturbance by reducing a predefined cost function associated with the power system performance.

When more induction generator based wind farms are integrated into the power system, the system voltage dips and stability problems may arise due to the draw of reactive power by induction generators. The power system short-circuit event induced wind turbine trips could result in power imbalance and lead to power system instability. This thesis studies the influence of two compensation techniques on the wind turbine low voltage ride-through (LVRT) capability. One of which is based on a parallel compensation by a static synchronous compensator (STATCOM), and the other one is a series compensation by a dynamic voltage restorer (DVR). In this study, Matlab tools and models are used to simulate an active-stall controlled fixed-speed induction generator connected to a power system. Two system configurations are used to simulate three phase faults and compare the improvement of wind turbine LVRT capability due to the two studied compensation techniques. Simulation results indicate that wind turbine compensated by DVR would have better LVRT performance than that by STATCOM in dealing with the low voltage situations due to system faults.

A solar and diesel-wind hybrid power control systems is proposed in the thesis. The system consists of solar power, wind power, diesel-engine, a static synchronous compensator and an intelligent power controller. MATLAB/Simulink was used to build the dynamic model and simulate the solar and diesel-wind hybrid power system. A static synchronous compensator was used to supply reactive power and regulate the voltage of the hybrid system. To achieve a fast and stable response for the real power control, an intelligent controller was proposed, which consists of the Radial Basis Function Network (RBFN) and the Elman Neural Network (ENN) for maximum power point tracking (MPPT). The pitch angle control of wind power uses ENN controller, and the output is fed to the wind turbine to achieve the MPPT. The solar system uses RBFN, and the output signal is used to control the DC / DC boost converters to achieve the MPPT.

In recent year, the awareness of environmental protection has made the power dispatch problem not necessarily economy-oriented. This thesis proposed the application of Particle Swarm Optimization (PSO) algorithm to solve the Unit Commitment (UC) problem for 24 hours with maximum profit in the power and carbon market. Optimal Power Flow (OPF) is used to solve the UC problem for the interconnected power network that is comprised of three independent areas to optimize the dispatching strategy. The UC problem must satisfy the constraints of the load demand, generating limits, minimum up/down time, ramp rate limits, and also the limits of power flow, buses voltage and transmission line capacity. The other objective of this thesis is to employ the Static Synchronous Series Compensator (SSSC) to integrate with OPF based on Equivalent Current Injection (ECI) power flow model, and install it at interconnected lines between each independent area controlling the power flow to reduce emission. In order to avoid the local optimality problem, this thesis proposed the utilization of the Multiple Particle Swarm Optimization (MPSO), which can quickly reach the optimal solution with a better performance and accuracy. The Independent Power Producer (IPP) can get the maximum profit with installed SSSC from the power and carbon trading with the calculation of power wheeling expense and carbon forecasting data. Furthermore, it can also assess the need of participating in the trading market or not.

X-ray scatter in cone-beam computed tomography (CBCT) is known to reduce image quality by introducing image artifacts, reducing contrast, and limiting computed tomography (CT) number accuracy. The extent of the effect of x-ray scatter on CBCT image quality is determined by the shape and magnitude of the scatter distribution in the projections. A method to allay the effects of scatter is imperative to enable application of CBCT to solve a wider domain of clinical problems. The work contained herein proposes such a method. A characterization of the scatter distribution through the use of a validated Monte Carlo (MC) model is carried out. The effects of imaging parameters and compensators on the scatter distribution are investigated. The spectral frequency components of the scatter distribution in CBCT projection sets are analyzed using Fourier analysis and found to reside predominately in the low frequency domain. The exact frequency extents of the scatter distribution are explored for different imaging configurations and patient geometries. Based on the Fourier analysis it is hypothesized the scatter distribution can be represented by a finite sum of sine and cosine functions. The fitting of MC scatter distribution estimates enables the reduction of the MC computation time by diminishing the number of photon tracks required by over three orders of magnitude. The fitting method is incorporated into a novel scatter correction method using an algorithm that simultaneously combines multiple MC scatter simulations. Running concurrent MC simulations while simultaneously fitting the results allows for the physical accuracy and flexibility of MC methods to be maintained while enhancing the overall efficiency. CBCT projection set scatter estimates, using the algorithm, are computed on the order of 1-2 minutes instead of hours or days. Resulting scatter corrected reconstructions show a reduction in artifacts and improvement in tissue contrast and voxel value accuracy.

X-ray scatter in cone-beam computed tomography (CBCT) is known to reduce image quality by introducing image artifacts, reducing contrast, and limiting computed tomography (CT) number accuracy. The extent of the effect of x-ray scatter on CBCT image quality is determined by the shape and magnitude of the scatter distribution in the projections. A method to allay the effects of scatter is imperative to enable application of CBCT to solve a wider domain of clinical problems. The work contained herein proposes such a method. A characterization of the scatter distribution through the use of a validated Monte Carlo (MC) model is carried out. The effects of imaging parameters and compensators on the scatter distribution are investigated. The spectral frequency components of the scatter distribution in CBCT projection sets are analyzed using Fourier analysis and found to reside predominately in the low frequency domain. The exact frequency extents of the scatter distribution are explored for different imaging configurations and patient geometries. Based on the Fourier analysis it is hypothesized the scatter distribution can be represented by a finite sum of sine and cosine functions. The fitting of MC scatter distribution estimates enables the reduction of the MC computation time by diminishing the number of photon tracks required by over three orders of magnitude. The fitting method is incorporated into a novel scatter correction method using an algorithm that simultaneously combines multiple MC scatter simulations. Running concurrent MC simulations while simultaneously fitting the results allows for the physical accuracy and flexibility of MC methods to be maintained while enhancing the overall efficiency. CBCT projection set scatter estimates, using the algorithm, are computed on the order of 1-2 minutes instead of hours or days. Resulting scatter corrected reconstructions show a reduction in artifacts and improvement in tissue contrast and voxel value accuracy.

Thesis (MEng)--Stellenbosch University, 2004. / ENGLISH ABSTRACT: This thesis presents the design of a DSP based controller system for an auxiliary converter for generic energy storage connection to a line-interactive power compensator. The aim is to utilize a wide range of energy storage systems and incorporate them into the existing power compensator. This was done by adding a second converter to the system. The new controller was developed to control this converter and thereby the energy storage. A study was done on lead acid batteries, flow batteries and flywheels in order to gain a basic understanding of these systems and identify the requirements for their control. Based on these findings, a new DSP based controller and measuring system was developed. To test the new system a dc to dc converter was implemented for the control of lead acid and flow batteries. A dc to ac converter was also tested for the control of flywheel energy storage. / AFRIKAANSE OPSOMMING: Die tesis handeloor die ontwerp van'n DSP gebaseerde beheerstelsel vir 'n Iyninteraktiewe on-onderbreekbare kragbron met drywings-kompensasie met 'n hulp omsetter vir algemene enegiestoor koppeling. The doel is om 'n wye verskeindheid energie store in die huidige drywings elektroniese stelsel te inkorporeer. 'n Tweede omstetter is by die stelsel gevoeg om die energie stelsel te beheer. 'n Nuwe beheerder is ontwikkel om die omsetter te beheer en daardeur die energie stoor. Loodsuur batterye, vloei batterye en vliegwiele is ondersoek om 'n basiese begrip te vorm en te identifiseer wat nodig is vir die beheer van sulke energie store. Die nuwe DSP beheerder en meetstelsel is ontwikkel gebaseer op hierde ondervindings. Om die nuwe beheerstelsel te toets is 'n gs na gs omsetter geïmplementeer vir die beheer van loodsuur en floei batterye. Die omsetter is geherkonfigureer na 'n gs na ws opstelling en getoets. Die konfigurasie word gebruik vir die beheer van vliegwiele.

Master of Science / Department of Electrical and Computer Engineering / Shelli Starrett / Variability in power systems is increasing due to pushing the system to limits for economic purposes, the inclusion of new energy sources like wind turbines and photovoltaic, and the introduction of new types of loads such as electric vehicle chargers. In this new environment, system monitoring and control must keep pace to insure system stability and reliability on a wide area scale. Phasor measurement unit technology implementation is growing and can be used to provide input signals to new types of control. Fuzzy logic based power system stabilizer (PSS) controllers have also been shown effective in various studies. This thesis considers several choices of input signals, composed assuming phasor measurement availability, for fuzzy logic-based controllers. The purpose of the controller is to damp power systems’ low frequency oscillations. Nonlinear transient simulation results for a 4-machine two-area system and 50 machine system are used to compare the effects of input choice and controller type on damping of system oscillations. Reactive power in the system affects voltage, which in turn affects system damping and dynamic stability. System stability and damping can be enhanced by deploying SVC controllers properly. Different types of power system variables play critical role to damp power swings using SVC controller. A fuzzy logic based static var compensator (SVC) was used near a generator to damp these electromechanical oscillations using different PMU-acquired inputs. The goal was again improve dynamic stability and damping performance of the system at local and global level. Nonlinear simulations were run to compare the damping performance of different inputs on the 50 machine system.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The growing global demand for electricity, increasing scarcity of the conventional energy sources and the need to reduce environmental impacts contributed to the pronounced development of new technologies based on clean and renewable energy sources. In this context, the wind power is the technology that has had a greater development in recent decades. This source has played an important role in the advancement of alternative sources in the world energy matrix, because the technology is already consolidated and is economically viable. Accompanying the technological development of the alternative sources, the induction generator has stood out among the generators with the technology best suited to generating electricity for isolated energy systems. With this motivation, this thesis proposes a new configuration for an isolated wind energy conversion system using variable speed turbines, in which the induction generator is electronically excited by means of a static compensator. Also, the generator is associated to a shunt active filter for compensation of non-linear and unbalanced loads, and with a battery bank for energy storage and DC bus stabilization. It is also discussed the main problems with the induction generator related to the magnetization characteristic and its dynamic model, and the proposed solutions to mitigate these problems. The results confirm the effectiveness of the proposed wind energy conversion system, showing its operational capability with variable load and variable turbine speed. Finally it is presented the main conclusions of this work and suggestions for future works in the same line of research. / O crescente aumento da demanda mundial de energia elétrica, a escassez cada vez maior das fontes convencionais de energia e a necessidade de redução dos impactos ambientais contribuíram para o acentuado desenvolvimento de novas tecnologias baseadas em fontes limpas e renováveis. Neste contexto, a energia eólica foi uma das tecnologias que mais se desenvolveu nas últimas décadas. Esta fonte tem desempenhado importante papel no avanço das fontes alternativas na matriz energética mundial, pois é uma tecnologia já consolidada e viável economicamente. Acompanhando o desenvolvimento tecnológico das fontes alternativas, o gerador de indução tem se destacado dentre as tecnologias de geradores mais adequadas para sistemas isolados de geração de energia elétrica. Com esta motivação, esta tese propõe uma nova configuração de conversão de energia eólica isolada, utilizando turbinas de velocidade variável, na qual o gerador de indução é excitado eletronicamente por meio de um compensador estático. Ainda mais, o gerador é associado a um filtro ativo shunt para a compensação de cargas não-lineares e desbalanceadas e a um banco de baterias para armazenamento de energia e estabilização do barramento cc, comum aos conversores. São discutidos os principais problemas relacionados ao gerador de indução para esta aplicação através da característica de magnetização e do seu modelo dinâmico, com as soluções propostas para mitigar estes problemas. Os resultados práticos e de simulação obtidos comprovam a efetividade da configuração de conversão de energia eólica proposta, mostrando a capacidade de operação do gerador com carga variável e velocidade variável da turbina. Por fim são apresentadas as principais conclusões e sugestões para trabalhos futuros na mesma linha de pesquisa.

Submitted by Felipe Augusto Ferreira de Almeida null (felipeafalmeida@gmail.com) on 2016-12-20T16:56:04Z No. of bitstreams: 1 DISSERTACAO_FELIPE_FINAL.pdf: 3666708 bytes, checksum: 18899b06cbe9a7091fd3f037b0656174 (MD5) / Approved for entry into archive by Felipe Augusto Arakaki (arakaki@reitoria.unesp.br) on 2016-12-22T10:53:09Z (GMT) No. of bitstreams: 1 almeira_faf_me_bauru.pdf: 3666708 bytes, checksum: 18899b06cbe9a7091fd3f037b0656174 (MD5) / Made available in DSpace on 2016-12-22T10:53:09Z (GMT). No. of bitstreams: 1 almeira_faf_me_bauru.pdf: 3666708 bytes, checksum: 18899b06cbe9a7091fd3f037b0656174 (MD5) Previous issue date: 2016-11-03 / Instituto Federal de Educação, Ciência e Tecnologia de São Paulo (IFSP) / Este trabalho aborda o emprego do Compensador Estático de Reativos (SVC) em redes de baixa tensão com geração distribuída de energia, tendo como objetivo a compensação de tensão e de fator de potência. Os principais distúrbios de qualidade de energia elétrica em baixa tensão, bem como as normas e os limites operacionais para os principais indicadores foram revisados, visando confrontação com as situações resultantes da integração do SVC no sistema elétrico. O SVC possui operação estabelecida como carga reativa controlável em sistemas de transmissão de energia, fazendo parte dos condicionadores da tecnologia FACTS (Flexible AC Transmission Systems). A exploração desta tecnologia em outro nível de tensão é o objetivo deste trabalho. As análises foram realizadas através de simulações computacionais, desenvolvidas no ambiente MATLAB, contemplando o desenvolvimento de modelos relacionados com aplicações de RCT (Reator Controlado a Tiristor), FC (Capacitor Fixo), do CCT (Capacitor Chaveado a Tiristor) e de um sistema de geração distribuída de energia elétrica trifásico em uma rede de baixa tensão a quatro fios. A metodologia para o dimensionamento dos elementos passivos dos condicionadores é apresentada, bem como a análise dos valores de potência reativa, fator de potência, distorções harmônicas e a avaliação da necessidade de utilização ou não de filtros passivos. A teoria da potência conservativa (TPC) é a metodologia utilizada para definição das grandezas elétricas e fatores de conformidade. Por fim, as características operacionais de um SVC aplicado a uma rede de baixa tensão a quatro fios são exploradas através dos resultados de simulação, com o objetivo de demonstrar a manutenção de suas características operacionais estabelecidas, no nível de baixa tensão, e apontar aspectos diferenciados quanto à regulação de fator de potência e da forma de tensão providos pelo SVC, para servir como informação de confronto frente a outras tecnologias comumente utilizadas neste nível de tensão. / This paper discusses the use of Static Var Compensator (SVC) on low voltage grids with distributed generation with the aim of voltage and power factor compensation. The main electrical power quality disturbances at low voltage as well as the rules and operational limits for the main indicators were reviewed aiming to confront the situations resulting from SVC integration in the electrical system. SVC has established operation as controllable reactive load in power transmission systems, being part of FACTS (Flexible AC Transmission Systems) technology conditioners. The application of this technology in another level of tension is the objective of this work. The analyzes were carried out through computer simulations developed in the MATLAB environment, including the development of models related to RCT (Thyristor Controlled Reactor), FC (Fixed Capacitor), CCT (Thyristor Switching Capacitor) and a threephase electric power distributed generation system in a four-wire low voltage grid. The methodology for the dimensioning of the passive elements of the conditioners is presented, as well as the analysis of the values of reactive power, power factor, harmonic distortions and the evaluation of the need to use passive filters. The conservative power theory (CPT) is the methodology used to define the electrical magnitudes and compliance factors. Finally, the operational characteristics of a SVC applied to a low-voltage four-wire network are exploited through the simulation results, in order to demonstrate the maintenance of its established operational characteristics at the low voltage level and to point out different aspects as well as the power factor regulation and voltage form provided by the SVC, to serve as confronting information.