A quantidade de falhas recentes em transformadores e suas consequências são alarmantes. As perdas econômicas, os riscos e a falta de confiabilidade decorrentes são inaceitáveis. Face à gravidade da situação um conjunto de recomendações de implementação imediata é estabelecido. O objetivo é assegurar a continuidade da operação confiável dos transformadores em serviço e garantir a especificação segura de transformadores novos. As recomendações envolvem o dimensionamento de enrolamentos levando em conta transitórios rápidos e ressonâncias internas; auditoria e controle de projetos; formas de ondas de ensaio mais representativas de fenômenos transitórios não convencionais; ações associadas à condições de operação; monitoração contínua e aplicação de filtros para a proteção frente a transitórios rápidos. Estas recomendações apoiam-se em análises cuidadosas das condições de projeto, fabricação e ensaios de transformadores complementadas pela análise teórica e de campo de falhas recentes em grandes transformadores de alta tensão. Vários modos de falhas dielétricas de transformadores estão associados a transitórios eletromagnéticos não convencionais. É analisada a modelagem de enrolamentos para transitórios de alta frequência. Os resultados de simulações aplicadas a alguns transformadores de alta tensão são apresentados. A aplicação das recomendações apresentadas deverá resultar na redução de falhas em grandes transformadores de alta tensão. / The number of transformer failures and its consequences are of very high concern. The associated economic losses, risks to life and environment and reduced reliability level are not acceptable any more. From this situation, a set of recommendation with possible immediately implementation is established. The objective is to give a reliable continuous operation condition to the remaining transformers in service and guarantee a safe specification for new transformers. The recommendations are including the dimensioning of transformer windings related to very fast transients and internal resonance; design review; dielectric test waves more representatives of non conventional transients phenomena; operation conditions; continuous monitoring and application of filter as protection device against very fast transients. These recommendations are supported by deep analysis of design conditions, manufacturing and test of power transformers complemented with theoretic and on site analysis of recent failures of high voltage large power transformers. Several dielectric failures modes of transformers are associated with non conventional electromagnetic transients. From this, it is included a modeling of transformer windings for high frequency transients. The results of simulations applied to high voltage transformers are presented. The application of the presented recommendations shall give as result a reduction in number of failures of large power transformers.

Neste trabalho é apresentada a solução das equações que regem o escoamento unidimensional não permanente em dutos sob pressão por meio do esquema numérico de MacCormack, esquema este, já empregado nos escoamentos transientes em condutos livres. É apresentada, também, a comparação dos resultados obtidos com a aplicação do esquema de MacCormack em relação aos obtidos através do método das características, na solução de transientes hidráulicos em condutos forçados. / This dissertation introduces the MacCormack numerical scheme for the solution of the one-dimensional unsteady pressurized flow equations, this scheme is already employed for solving open-channel transient flows. It is presented, also, the comparison of the results obtained from the application of the MacCormack numerical scheme with the obtained using the Method of the Characteristics, in the solution of hydraulic transient in pressurized flow.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by José Converti. / Ph.D.

The motivation for this thesis is to devise a simple model of transient dynamics in neural networks. Neural circuits are capable of performing many computations without reaching an equilibrium, but instead through transient changes in activity. Thus, having a good model for transient activity is important. In particular, this thesis focuses on a firing-rate description of neural activity. Firing rates offer a convenient simplification of neural activity, and have been shown experimentally to convey information about stimuli and behavior. This work begins by review the philosophy of modeling firing rates, as well as the problems that go with it. It examines traditional approaches to modeling firing rates, and in particular how common assumptions lead to a model that fails to capture transient dynamics. Chapter 2 applies a traditional model of firing rates in order to gain insight into properties of cortical circuitry. In collaboration with the lab of David Ferster at Northwestern University, we found that surround suppression in cat primary visual cortex is mediated by a withdrawal of excitation in the cortical circuit. In theoretical work, we find that this behavior can only arise if excitatory recurrence alone is strong enough to destabilize visual responses but feedback inhibition maintains stability. Chapter 3 reviews concepts and literature related to the dynamics of large networks of spiking neurons. Population density approaches are common for describing the dynamics of networks of spiking neurons. These approaches allow for a rigorous approach to relate the dynamics of individual neurons to the population firing rate. Chapter 4 explores a method for accurately approximating the firing-rate dynamics of a population of spiking neurons. We describe the population by the probability density of membrane potentials, so the dynamics are governed by a Fokker-Planck equation. Using a spiking model with periodic boundary conditions, we write the Fokker-Planck dynamics in a Fourier basis. We find that the lowest Fourier modes dominate the dynamics. Chapter 5 presents a novel rate model that successfully captures synchronous dynamics. As in the previous chapter, we invoke an approximation to the dynamics of a population of spiking neurons in order to develop a firing-rate model. Our approach derives from an eigenfunction expansion of a Fokker-Planck equation, which is a common approach to solving such problems. We find that a very simple approximation turns out to be surprisingly accurate. This approximation allows us to write a closed-form expression for the firing rate that resembles the equations for a damped harmonic oscillator. Finally, chapter 6 uses the formalism derived in the previous chapter to analyze activity in a large randomly-connected network of neurons. Comparing this large spiking network to a network of two coupled rate units, we find that the firing rate network gives a good approximation to the time-varying activity of a spiking network across a wide range of parameters. Perhaps most surprisingly, we also find that the rate network can approximate the phase diagram of the spiking network, predicting the bifurcation line between synchronous and asynchronous states.

Neste trabalho é apresentada a solução das equações que regem o escoamento unidimensional não permanente em dutos sob pressão por meio do esquema numérico de MacCormack, esquema este, já empregado nos escoamentos transientes em condutos livres. É apresentada, também, a comparação dos resultados obtidos com a aplicação do esquema de MacCormack em relação aos obtidos através do método das características, na solução de transientes hidráulicos em condutos forçados. / This dissertation introduces the MacCormack numerical scheme for the solution of the one-dimensional unsteady pressurized flow equations, this scheme is already employed for solving open-channel transient flows. It is presented, also, the comparison of the results obtained from the application of the MacCormack numerical scheme with the obtained using the Method of the Characteristics, in the solution of hydraulic transient in pressurized flow.

O presente trabalho versa sobre o termo de atrito em escoamento transitório em condutos forçados, partindo de uma modelação matemática da tensão de cisalhamento transiente, usando conceitos do princípio da entropia máxima. Usa-se a modelação deste termo de atrito na análise do transitório hidráulico, comparando-se com dados experimentais já publicados de um sistema Reservatório-Tubo-Válvula. / This report contains a friction term for hydraulic transient in closed conduits. We use a mathematical model for the shear stress, derived by principle of maximum entropy. Then, we use this friction term for an analysis in a system reservoir-tube-valve, comparing our results with anothers already published in the literature.

A quantidade de falhas recentes em transformadores e suas consequências são alarmantes. As perdas econômicas, os riscos e a falta de confiabilidade decorrentes são inaceitáveis. Face à gravidade da situação um conjunto de recomendações de implementação imediata é estabelecido. O objetivo é assegurar a continuidade da operação confiável dos transformadores em serviço e garantir a especificação segura de transformadores novos. As recomendações envolvem o dimensionamento de enrolamentos levando em conta transitórios rápidos e ressonâncias internas; auditoria e controle de projetos; formas de ondas de ensaio mais representativas de fenômenos transitórios não convencionais; ações associadas à condições de operação; monitoração contínua e aplicação de filtros para a proteção frente a transitórios rápidos. Estas recomendações apoiam-se em análises cuidadosas das condições de projeto, fabricação e ensaios de transformadores complementadas pela análise teórica e de campo de falhas recentes em grandes transformadores de alta tensão. Vários modos de falhas dielétricas de transformadores estão associados a transitórios eletromagnéticos não convencionais. É analisada a modelagem de enrolamentos para transitórios de alta frequência. Os resultados de simulações aplicadas a alguns transformadores de alta tensão são apresentados. A aplicação das recomendações apresentadas deverá resultar na redução de falhas em grandes transformadores de alta tensão. / The number of transformer failures and its consequences are of very high concern. The associated economic losses, risks to life and environment and reduced reliability level are not acceptable any more. From this situation, a set of recommendation with possible immediately implementation is established. The objective is to give a reliable continuous operation condition to the remaining transformers in service and guarantee a safe specification for new transformers. The recommendations are including the dimensioning of transformer windings related to very fast transients and internal resonance; design review; dielectric test waves more representatives of non conventional transients phenomena; operation conditions; continuous monitoring and application of filter as protection device against very fast transients. These recommendations are supported by deep analysis of design conditions, manufacturing and test of power transformers complemented with theoretic and on site analysis of recent failures of high voltage large power transformers. Several dielectric failures modes of transformers are associated with non conventional electromagnetic transients. From this, it is included a modeling of transformer windings for high frequency transients. The results of simulations applied to high voltage transformers are presented. The application of the presented recommendations shall give as result a reduction in number of failures of large power transformers.

Esta dissertação descreve um simulador em tempo real para aplicação em testes de reguladores de velocidade de hidrelétricas. O simulador é implementado num hardware que permite a conexão física de todos os sinais que o regulador teria na usina. As dimensões e o perfil hidráulico da usina, dados das máquinas e unidade de força hidráulica são inseridos através de tabelas de fácil entendimento pelo usuário Os transientes hidráulicos no conduto forçado são simulados usando o método das características e levam em conta a influência da turbina. A unidade hidráulica e os atuadores são modelados incluindo suas não-linearidades. Além dos testes de fábrica, este simulador pode ser utilizado para análise preliminar de transientes hidráulicos, treinamento de operadores e como plataforma de testes para novas estratégias de controle da turbina. / This work describes a real-time simulator to be used in hydroturbine speed governor tests. The simulator is implemented using a hardware configuration which allows the physical connection to the governor under test like it would be in the hydropower plant. The conduit dimensions, hydraulic scheme, machine and hydraulic power unit data are inserted into the simulator by user friendly input tables. The hydraulic transients in the penstock are simulated using the method of characteristics and take into account the turbine influence. The electrohydraulic interface, servomotor and hydraulic power unit are simulated including the nonlinearities of valves and servomotors. Besides the factory tests, this simulator can also be applied for a preliminary analysis of hydraulic transients, operator training and platform for testing new turbine control strategies.

The purpose of this study was to determine whether the compression circuit of a hearing aid could be activated by a high frequency biasing tone such that its output for a transient stimulus could be made to approximate that of a continuous tone alone. Sufficient compression activation by a transient stimulus would mean that this type of hearing aid could be used in obtaining aided ABR measurements, since transient stimuli are commonly used for this procedure. Four hearing aids were used, and transient or continuous stimuli were introduced either alone or in combination with an 8 or 10 kHz biasing tone. The biasing tone was either held at one constant level or varied in intensity along with the transient or continuous signal.

"August 2002" / Bibliography: leaves 219-228. / xxix, 255 p. : ill., photos. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Civil and Environmental Engineering. 2003