Este trabalho apresenta uma proposta para cálculo da correção do fator de construção de núcleos de transformadores de potência, devido à influência das perdas na região das juntas magnéticas. Esse fator permite corrigir o valor de perdas em vazio no núcleo, obtido das curvas do fabricante das chapas de aço-silício, e sua determinação, até o momento, tem sido baseada, sobretudo, em estimativas empíricas e estatísticas. A correção proposta para esse fator é baseada no cálculo das perdas magnéticas na região das juntas, cálculo esse obtido a partir de uma simulação computacional do transformador pelo Método de Elementos Finitos. Com esse fator de correção, foi possível melhorar a precisão do cálculo de perdas em vazio, aproximando-o mais dos valores experimentais. Foram estudados dois tipos de juntas magnéticas, com e sem step-lap, e três dimensões de entreferro; em seguida, foi analisada a influência desses parâmetros no fator de construção. As simulações foram realizadas por um programa comercial que utiliza o Método de Elementos Finitos em duas dimensões. / This work presents a suggestion to calculate a correction factor of building factor in a core of power transformers, due to influence of losses in the magnetic junctions. This factor allows correcting the value of no load loss in the core, obtained from steel manufacturers, and their determination, up to now, it has been based, especially, in empirical and statistics estimates. The correction suggested to this factor is based in magnetic losses calculations in junction region, calculation which was obtained from a computer simulation of the transformer by Finite Element Method. Having this correction factor, could the accuracy of the calculation of no load loss, be improved, setting of the experimental value closer. Two types of magnetic joints, with and without step-lap, and three of gap were studied; then, it was examined the influence of these parameters in the building factor. The simulations were run by commercial software which uses finite element method in two dimensions.

Transformadores de potência são equipamentos essenciais no sistema elétrico de potência, alterando os níveis de tensão para interligar os sistemas de geração, transmissão e distribuição de energia elétrica. Neste trabalho são relacionados e descritos os principais modos de falha normalmente verificados em transformadores, associados ao levantamento estatístico que compõe um banco de dados elaborado a partir de perícias realizadas entre os anos de 2.000 e 2.008 para companhias seguradoras. É apresentada e desenvolvida uma análise de falhas verificadas em cerca de uma centena de transformadores com diferentes tipos de aplicação, classes de tensão e níveis de potência. O objetivo do estudo é contribuir com um melhor entendimento de causas de falhas e os tipos de transformadores mais suscetíveis a cada uma delas. / Power transformers are essential equipment in the electric system by changing the voltage levels in order to connect generation, transmission and distribution systems of electric energy. In this study are related and described the most common failure modes in transformers, associated with the statistical statement that makes up a database developed from expertise held between the years 2000 and 2008 for insurance companies. It is presented and developed an analysis of failures found in around a hundred transformers with different types of application, classes of voltage and power levels. The objective of the study is to contribute to a better understanding of the causes of failures and the transformers types most susceptible to each one.

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.

One of the most important industrial processes is heat transfer, carried out by heat exchangers in single and multiphase flow applications. Despite the existence of well-developed theoretical models for different heat transfer mechanisms, the expanding need for industrial applications requiring the design and optimization of heat exchangers, has created a solid demand for experimental work and effort. This thesis concerns the use of numerical approaches to analyze and optimize heat transfer and fluid flow in power generation industry, with emphasis on pin fin technology. This research begins with a review on heat transfer characteristics in surfaces with pin fins. Different pin fins shapes with various flow boundaries were studied, and thermal and hydraulic performances were investigated. The impact of parameters such as inlet boundary conditions, pin fin shapes, and duct cross-section characteristics on both flow and heat transfer were examined. Two important applications in power generation industry were considered for this study: power transformer cooling, and condenser for CO2 capturing application in oxy-fuel power plants. Available experimental data and correlations in the literature have been used for models validation. For each case, a model based on current configuration was built and verified, and was then used for optimization and new design suggestions. All numerical modeling was performed using commercial CFD software. A basic condenser design was suggested and examined, supplemented by the use of pin fin technology to influence the condensation rate of water vapour from a CO2/H2O flue gas flow. Moreover an extensive review of numerical modeling approaches concerning this condensation issue was conducted and presented. The analysis results show that the drop-shaped pin fin configuration has heat transfer rates approximating those of the circular pin configuration, and the drop-shaped pressure losses are less than one third those of the circular. Results for the power transformer cooling system show those geometrical defects in the existing system are easily found using modeling. Also, it was found that the installation of pin fins in an internal cooling passage can have the same effect as doubling the radiator’s height, which means a more compact cooling system could be designed. Results show that a condensation model based on boundary layer theory gives a close value to experimental correlations. Considering a constant wall temperature, any increase in CO2 concentration results in lower heat transfer coefficients. This is a subsequence of increased diffusivity resistance between combustion gas and condensing boundary layer. Also it was shown that sensitivity of heat transfer rate to inlet temperatures and velocity values decreased when these parameters increased. The application of numerical methods concerning the condensation process for CO2 capturing required significant effort and running time as the complexity of multiphase flow was involved. Also data validation for the CO2/H2O condenser was challenging since this is quite a new application and less experimental data (and theoretical correlations) exist. However, it is shown that models based on numerical approaches are capable of predicting trends in the condensation process as well as the effect of the non-condensable CO2 presence in the flue gas. The resulting data, conclusions, applied methodology can be applied to the design and optimization of similar industrial heat exchangers, such as oil coolers which are currently working at low efficiency levels. It can also be used in the design of electronic components, cooling of turbine blades, or in other design applications requiring high heat flux dissipation. Finally, the finding on water vapour condensation from a binary mixture gas can be referenced for further research and development in this field.

A power transformer is one of the most important and expensive components in any power system. Power transformers can be exposed to a wide variety of abnormal conditions and faults. Internal turn-to-turn faults are the most difficult types of faults to detect within the power transformer. The IEEE Standards documents have revealed that there is no one standard way to protect all power transformers against minor internal faults such as turn-to-turn faults and at the same time to satisfy basic protection requirements: sensitivity, selectivity, and speed.<p> This thesis presents a new, simple and efficient protection technique which is based on negative sequence currents. Using this protection technique, it is possible to detect minor internal turn-to-turn faults in power transformers. Also, it can differentiate between internal and external faults. The discrimination is achieved by comparing the phase shift between two phasors of total negative sequence current. The new protection technique is being studied via an extensive simulation study using PSCAD®/EMTDC 1 software in a three-phase power system and is also being compared with a traditional differential algorithm.<p> Relay performance under different numbers of shorted turns of the power transformer, different connections of the transformer, different values of the fault resistances, and different values of the system parameters was investigated. The results indicate that the new technique can provide a fast and sensitive approach for identifying minor internal turn-to-turn faults in power transformers.

A power transformer is one of the most important and expensive components in any power system. Power transformers can be exposed to a wide variety of abnormal conditions and faults. Internal turn-to-turn faults are the most difficult types of faults to detect within the power transformer. The IEEE Standards documents have revealed that there is no one standard way to protect all power transformers against minor internal faults such as turn-to-turn faults and at the same time to satisfy basic protection requirements: sensitivity, selectivity, and speed.<p> This thesis presents a new, simple and efficient protection technique which is based on negative sequence currents. Using this protection technique, it is possible to detect minor internal turn-to-turn faults in power transformers. Also, it can differentiate between internal and external faults. The discrimination is achieved by comparing the phase shift between two phasors of total negative sequence current. The new protection technique is being studied via an extensive simulation study using PSCAD®/EMTDC 1 software in a three-phase power system and is also being compared with a traditional differential algorithm.<p> Relay performance under different numbers of shorted turns of the power transformer, different connections of the transformer, different values of the fault resistances, and different values of the system parameters was investigated. The results indicate that the new technique can provide a fast and sensitive approach for identifying minor internal turn-to-turn faults in power transformers.

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.

Les transformateurs de puissance sont des éléments clés des systèmes électriques. Leurs défaillances sont très coûteuses, principalement à cause de la non-disponibilité du service électrique qu'elles entraînent. L'évaluation rapide et précise de défauts internes des transformateurs est en conséquence un facteur clé d'une exploitation efficace et sûre. Un tel diagnostic est généralement établi par un expert humain qui fait corréler différents types d'informations telles que des résultats d'essais électriques ou chimiques, le déclenchement de protections ou l'historique de l'appareil… Cette thèse présente une méthode d'aide au diagnostic de défauts originale qui reprend, en les formalisant, la démarche et la capitalisation d'expérience de l'expert. Les informations disponibles sur le transformateur à étudier sont mises en correspondance avec le système proposé de manière systématique. Des hypothèses de défaut sont alors formulées et un degré de confiance calculé pour chacune d'elle. Pour améliorer la confiance en certaines de ces hypothèses la méthode recherche, et propose, de renseigner des informations utiles pouvant améliorer le diagnostic. Il progresse ainsi jusqu'à ce que la confiance d'au moins une hypothèse soit suffisante pour l'utilisateur. Le système peut de plus évoluer efficacement dans le temps en prenant facilement en compte de nouveaux types d'essais ou de nouvelles informations pouvant être discriminants dans un diagnostic, et ainsi améliorer les diagnostics futurs automatiquement. / Power transformers are keys components to the electrical systems. Its failures are very expensive mostly by causing long electrical service unavailability. Quick and accurate assess of transformer's internal faults is consequently a key issue for an efficient and safe service life. Such diagnosis is mainly led by a human expert, due to his ability to link together many different kind of information as electrical or chemical tests, tripped protections or history of the apparatus… This thesis presents an original method of diagnosis aid that behaves somehow like an expert, and capitalizes his experience, by formalizing it. The available information on a particular transformer are used to search any similarities within the developed system, on a systematic base. Then faults hypothesis are proposed and according confidences are calculated. To improve those hypothesis' confidences the method will propose to inform new useful information that could improve the diagnosis. Then it continues up until at least one hypothesis confidence is high enough for the user. This system could easily evolve over time by taking into account new kind of tests or new information that could be significant in any diagnosis, and then improve future diagnosis automatically.

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 propÃe um novo mÃtodo de diagnÃstico de falhas incipientes em equipamentos de potÃncia com isolamento a Ãleo baseado em regras interpretÃveis extraÃdas a partir das concentraÃÃes dos gases presentes e dissolvidos no Ãleo isolante e de relaÃÃes entre essas concentraÃÃes nÃo exploradas por outros mÃtodos. Utilizando uma Ãrvore de decisÃo (Decision Tree - DT) com modificaÃÃes na dimensÃo dos atributos de entrada para a obtenÃÃo de regras, o mÃtodo proposto: (i) permite que o usuÃrio conheÃa as regras que geraram o diagnÃstico, (ii) contorna limitaÃÃes comuns a outros mÃtodos (ex.: existÃncia de regiÃes de ânÃo-diagnÃsticoâ, existÃncia de mais de um diagnÃstico possÃvel) e (iii) apresenta percentual elevado de acerto no diagnÃstico de dados de DGA (Dissolved Gas Analysis) disponÃveis na literatura. O mÃtodo proposto à comparado com outros mÃtodos de DGA descritos em normas: GÃs Chave, Rogers, DÃrnenburg e TriÃngulo de Duval. Na comparaÃÃo dos mÃtodos foram utilizados bancos de dados com informaÃÃes publicadas, entre eles os dados do IEC TC 10 que apresentam informaÃÃes e diagnÃstico de um comità de especialistas. Os Ãndices de acerto observados com o mÃtodo proposto estiveram acima de 94%. Comparativamente, o MÃtodo do TriÃngulo de Duval apresenta Ãndices de acerto entre 83% e 88%. O mÃtodo tambÃm foi avaliado com relaÃÃo à sensibilidade na ocorrÃncia de erros nas medidas de laboratÃrio, apresentando resultados comparativamente superiores (Ãndice de acerto de 83% em comparaÃÃo com 78% do MÃtodo do TriÃngulo de Duval). TambÃm foi desenvolvido neste trabalho um mÃtodo para o diagnÃstico simplificado de equipamentos de potÃncia com isolamento a Ãleo utilizando dados fÃsico-quÃmicos â os Ãndices de acerto obtidos foram inferiores aos alcanÃados com o DGA proposto, porÃm prÃximos aos observados em mÃtodos de DGA citados na literatura, como Rogers IEEE C57.104 â 2008, com a vantagem dos dados nÃo dependerem da disponibilidade de um cromatÃgrafo. / This work proposes a new method for diagnosis of incipient faults in power equipment with insulation oil. It consists on a set of interpretable rules extracted from the concentrations of the dissolved gases in insulating oil, and the relationships between these concentrations. Using a Decision Tree (DT) with changes in the size of the input attributes for obtaining rules, the proposed method: (i) allows the user to know the rules that generated the diagnosis, (ii) bypasses limitations common to other methods (e.g. the existence of "no diagnosis" regions, and the occurrence of more than one possible diagnosis) and (iii) the accuracy in diagnosis of DGA (Dissolved Gas Analysis) data available in the literature. The proposed method is compared with other DGA standard methods: Gas Key, Rogers, Dornenburg and DuvalÂs Triangle. The performed comparison considered databases available in technical literature, including data from the IEC TC 10. The observed accuracy of the proposed method is above 94%. Comparatively, the DuvalÂs Triangle method presents accuracy between 83% and 88% for the considered databases. The method was also evaluated with respect to sensitivity to measurements errors of laboratory â the results are very promising, the proposed method shows a hit rate of 83% in comparison with a hit rate of 78% for the DuvalÂs Triangle method. This work has also been developed a method for simplified diagnostic of power equipments with insulated oil using physico-chemical data - the hit rates obtained were lower than those achieved with the proposed DGA, however close to those observed in DGA methods cited in the literature, as Rogers IEEE C57.104 - 2008, with the advantage of the data not depend on the availability of a gas chromatograph.