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Natural lightning strikes induce impulsive overvoltages on transmission lines and its terminal equipment. These overvoltages may cause failures in insulation mechanisms of electrical devices in the power system. It is important to test the insulation strength of a device against these impulsive overvoltages. Usually, Marx generators are used to generate impulse waveforms for testing purposes. A novel approach is proposed to obtain resistor settings of a Marx generator for impulse testing of power transformers. This approach enables us to overcome most of the major challenges in the commonly used trial-and-error method, including excessive time consumption and potential damage to the transformer. The proposed approach uses the frequency response of the transformer to synthesize a circuit model. Then, a genetic algorithm based optimization-enabled electromagnetic transient simulation approach is used to obtain the resistor settings. The proposed approach is validated by a real impulse test conducted on a three phase power transformer.

Power systems in many countries are stressed towards their stability limit. If these stable systems experience any unexpected contingencies, or disturbances, there is a significant risk of instability, which may lead to wide-spread blackout. Existing methods to minimize the risk of stability and excessive frequency decline; need to be redeveloped to address these new challenges. This research project will develop a new emergency control methodology, which can more effectively prevent power system frequency and voltage instability under emergency conditions. Frequency and voltage instability are two major concerns in power system operation. The primary aim of this project is to develop new optimal load shedding techniques, which are able to better address various voltage and frequency instability issues for power systems emergency control purpose. In this thesis, new approach of load shedding for frequency and voltage stability are presented. For the load shedding to prevent frequency collapse, System Frequency Respond – Under Frequency Load Shedding (SFR-UFLS) from the previous approach has been redeveloped to compute an optimal load shedding scheme. The limitation of previous optimal load shed method is that they only shed load following one particular contingency event. As an improvement of this method, we developed a technique that protects against a range of contingencies. For the load shedding to prevent voltage collapse, The proposed method is then tested on the 39-bus New England test system. Generators are of different importance to the system in terms of voltage stability. It is essential to investigate generators’ impact on system voltage stability. The theory of the normal forms of diffeomorphism is used to analyze the power flow equations, and then nonlinear active participation factor is obtained and is used to determine the influence of generators on voltage stability. By using this method, the nonlinearity of power systems can be taken into consideration while the computational efficiency is maintained. Therefore, the impact of generators can be measured with more accuracy even for the cases in which the system is characterized with strong nonlinearity. In order to show the validity of the proposed method, the IEEE 14-bus test system and the New England 39-bus power system are used as case studies. The steady-state voltage stability index verifies the proposed method. The results show that nonlinear active participation factor can describe the characteristics even when power systems are operating at a highly stressed condition.

Power systems in many countries are stressed towards their stability limit. If these stable systems experience any unexpected contingencies, or disturbances, there is a significant risk of instability, which may lead to wide-spread blackout. Existing methods to minimize the risk of stability and excessive frequency decline; need to be redeveloped to address these new challenges. This research project will develop a new emergency control methodology, which can more effectively prevent power system frequency and voltage instability under emergency conditions. Frequency and voltage instability are two major concerns in power system operation. The primary aim of this project is to develop new optimal load shedding techniques, which are able to better address various voltage and frequency instability issues for power systems emergency control purpose. In this thesis, new approach of load shedding for frequency and voltage stability are presented. For the load shedding to prevent frequency collapse, System Frequency Respond – Under Frequency Load Shedding (SFR-UFLS) from the previous approach has been redeveloped to compute an optimal load shedding scheme. The limitation of previous optimal load shed method is that they only shed load following one particular contingency event. As an improvement of this method, we developed a technique that protects against a range of contingencies. For the load shedding to prevent voltage collapse, The proposed method is then tested on the 39-bus New England test system. Generators are of different importance to the system in terms of voltage stability. It is essential to investigate generators’ impact on system voltage stability. The theory of the normal forms of diffeomorphism is used to analyze the power flow equations, and then nonlinear active participation factor is obtained and is used to determine the influence of generators on voltage stability. By using this method, the nonlinearity of power systems can be taken into consideration while the computational efficiency is maintained. Therefore, the impact of generators can be measured with more accuracy even for the cases in which the system is characterized with strong nonlinearity. In order to show the validity of the proposed method, the IEEE 14-bus test system and the New England 39-bus power system are used as case studies. The steady-state voltage stability index verifies the proposed method. The results show that nonlinear active participation factor can describe the characteristics even when power systems are operating at a highly stressed condition.

Power systems in many countries are stressed towards their stability limit. If these stable systems experience any unexpected contingencies, or disturbances, there is a significant risk of instability, which may lead to wide-spread blackout. Existing methods to minimize the risk of stability and excessive frequency decline; need to be redeveloped to address these new challenges. This research project will develop a new emergency control methodology, which can more effectively prevent power system frequency and voltage instability under emergency conditions. Frequency and voltage instability are two major concerns in power system operation. The primary aim of this project is to develop new optimal load shedding techniques, which are able to better address various voltage and frequency instability issues for power systems emergency control purpose. In this thesis, new approach of load shedding for frequency and voltage stability are presented. For the load shedding to prevent frequency collapse, System Frequency Respond – Under Frequency Load Shedding (SFR-UFLS) from the previous approach has been redeveloped to compute an optimal load shedding scheme. The limitation of previous optimal load shed method is that they only shed load following one particular contingency event. As an improvement of this method, we developed a technique that protects against a range of contingencies. For the load shedding to prevent voltage collapse, The proposed method is then tested on the 39-bus New England test system. Generators are of different importance to the system in terms of voltage stability. It is essential to investigate generators’ impact on system voltage stability. The theory of the normal forms of diffeomorphism is used to analyze the power flow equations, and then nonlinear active participation factor is obtained and is used to determine the influence of generators on voltage stability. By using this method, the nonlinearity of power systems can be taken into consideration while the computational efficiency is maintained. Therefore, the impact of generators can be measured with more accuracy even for the cases in which the system is characterized with strong nonlinearity. In order to show the validity of the proposed method, the IEEE 14-bus test system and the New England 39-bus power system are used as case studies. The steady-state voltage stability index verifies the proposed method. The results show that nonlinear active participation factor can describe the characteristics even when power systems are operating at a highly stressed condition.

Thesis (M.S.)--Ohio University, August, 1995. / Title from PDF t.p.

Thesis (M.S.)--University of North Texas, Dec., 2008. / Title from title page display. Includes bibliographical references.

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

No novo contexto do setor elétrico brasileiro, as concessionárias passam a ter uma nova postura quanto à gestão. A energia elétrica, como produto, como negócio, deve levar lucro às empresas concessionárias. Começam a dar mais importância aos custos, entendendo que vários aspectos técnicos devem ser analisados, visando reduzir perdas substanciais. Assim sendo, o transformador de potência assume uma posição destacada, considerando sua posição estratégica no sistema elétrico, fundamental no processo de transporte e entrega de energia elétrica. Por este motivo vem despertando o interesse no desenvolvimento de estudos e pesquisas no sentido de aumentar sua vida útil. O método de Análise da Resposta em Frequência da impedância do transformador permite obter a “assinatura” do mesmo, a fim de verificar futuras degradações. Este método é adotado por várias empresas em todo o mundo, há mais de três décadas. Embora sua ampla aplicação entre muitas técnicas utilizadas na monitoração e análise de falhas em equipamentos, até agora não houve consenso na interpretação dos resultados obtidos por este método. A dificuldade de correlacionar parâmetros com prováveis falhas tem suscitado pesquisas para obtenção de resultados confiáveis e de interpretação fácil. Este trabalho objetiva determinar um modelo elétrico que represente a característica da resposta em frequência para fins de armazenamento de dados e simulações. A partir de dados reais do teste de resposta em frequência da impedância inseridos no modelo proposto, através de um programa computacional, foi verificada a confiabilidade nas respostas obtidas, comparadas com as reais. Os resultados obtidos asseguram que o modelo proposto é viável de ser aplicado. / In the new context of the Brazilian electric industry, the electric power provider companies have adopted a new administration posture. The electric power, as a product and business, should make the companies profitable. They have been paying more attention to the costs, .learning that many technical aspects should be analyzed in order to reduce substantial losses. Therefore, the power transformer takes an outstanding position, considering its strategic position in the electric system and being essential in the process of transport and delivery of the electric power. For this reason, there has been an interest in the development of studies and research in order to increase its useful life. The method of Frequency Response Analysis of impedance of the power transformer permits to obtain reference parameters of this one, in order to verify future degradations. This method has been adopted by many companies throughout the world for over three decades. Despite its wide application among several techniques used in the supervision and analysis of faults of equipments, there has been no consensus in the interpretation of the results obtained by this method so far. The difficulty of correlating the parameters with probable faults have increased research in order to obtain reliable results and of easy interpretation. This study aims to determine an electric model that represents the characteristic of the frequency response in order to store data and simulations. Starting from the real data of the FRA test inserted in the proposed model through a computer program, the reliability of the obtained responses were verified and compared with the real data. The obtained results assure that the proposed model is viable and can be applied.

Este trabalho consiste na aplicação de técnicas de identificação de parâmetros estruturais, especificamente massa, rigidez e amortecimento, à caixa acústica de um violão clássico. A abordagem experimental tradicional é adotada, com medição de aceleração em um ponto da estrutura e excitação proveniente de martelo instrumentado registrada em diferentes pontos. As funções de resposta em frequência são então traçadas para, em conjunto com um modelo analítico representativo dos primeiros graus de liberdade do acoplamento ar/estrutura, realizar-se o ajuste de curvas e posterior identificação dinâmica. Para tanto, esta última etapa foi tratada como um problema matemático de otimização não linear no qual se busca a minimização de uma função objetivo que contabiliza de alguma forma a diferença entre o resultado das medições e os valores previstos pelo modelo utilizado. Várias funções de erro (objetivo) e vários algoritmos foram utilizados, como mínimos quadrados não linear, simplex de Nealder-Mead, algoritmo genético padrão e enxame de partículas (particle swarm), entre outros, sendo este último o de melhor desempenho entre todos, quando aplicado em conjunto com a norma da diferença dos logaritmos das magnitudes ao quadrado. Assim, uma calibração com boa concordância entre dados experimentais e resultados teóricos foi estabelecida para o modelo proposto, sendo realizada também a verificação do ajuste através da comparação de um conjunto independente de dados, trazendo, desta forma, confiabilidade para posteriores cálculos das grandezas associadas ao comportamento dinâmico utilizando-se o modelo ajustado. / This work aims at identify structural parameters of a classical acoustic guitar’s resonant chamber by comparing theoretical and experimental frequency response functions. The quantities used to construct the mass, stiffness and damping matrices of an air/structure analytical model, which is representative of the first few modes of the body, are considered as project variables, and an impact vibration testing approach is used to obtain the measured data, with a roving instrumented hammer and an accelerometer attached to a predefined point of the body. Then, a curve-fit analysis is performed as a mathematical problem of non-linear optimization, wherein the objective function, which is to be minimized, somehow accounts for the difference between the measured data and the theoretically predicted values. Some different error (objective) functions and optimization algorithms, like non-linear least-squares, Nealder-Mead simplex, standard genetic algorithm and particle swarm, among others, were applied, and the latter yielded, together with the squared error norm, the best performance. Then, an updated model was achieved with fair agreement between analytical predictions and experimental results, verified using a validation data set, bringing reliability for further theoretical predictions.