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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Wake-Induced Oscillations in Cable Structures: Finite Element Approach

Snegovskiy, Dmitri 11 June 2010 (has links)
In this work we consider the overhead power transmission lines (OHL). Their specifics are related to the presence of cables (conductors) whose length between supporting towers may extend to dozens of thousand meters. The OHL components are exposed to a combination of natural actions wind, rain, ice / snow / frost deposits. Compared to other structural parts, conductors have the highest flexibility and very low structural self-damping (of the order of 0.1 % of critical damping or lower, depending on frequencies). They are among structural elements the most sensitive to these actions. Since early fifties the increased energy demand gave a rise to large construction of high-voltage and extra-high-voltage overhead lines equipped with bundled electrical conductors. For such arrangements there was noticed a kind of wind-induced oscillations originated by a zone of disturbed and retarded air flow (wake), that the cables located upwind(windward) cast onto the downwind (leeward) ones. The effect of this phenomenon called Wake-Induced Oscillations (WIO) resulted in fatigue damages of conductors, failures of insulator strings and cable suspension hardware and fatigue failures of spacers. There have been identified analogues to transmission lines WIO in other regular structures subjectto the cross-flow of viscous fluids (air, gas, water etc.): heat exchanger tubes, clusters of fuel rods of nnuclear reactors, groups of chimneys, buildings. Early works in this field relate to the aerodynamics of tandem and staggered twin struts to support the wings of biplanes and published by Pannell, Griffits and Coales in 1915. Other cable structures like suspenders in suspension bridges or stays in cable-stayed bridges may be also subject to wake-induced oscillations. In each of these cases, conditions of oscillations occurrence and structural response depend on cables specific mass and stiffness, kind of fixation, dimension scale versus fluid viscosity and velocity (Reynolds number) etc. The cables separation plays important role, as there are different kinds of wake interference especially when the cables are closely spaced. A number of research projects were entertained to study the wake-induced oscillations of different structures, which brought to development of analytical and experimental models and methods of protection against this phenomenon. A particular solution to overhead lines was found by unevenly distributing the spacers along the line span. To achieve that, no unique approach exists; virtually each grid company, or manufacturer of spacers proceeds with its own method. It may rely on different basis, either field experience or analytical study or a mixture of them. And, despite advances in numerical modelling of latest decades, few publications uncover phenomenological side of WIO. The issues of modelling WIO in a view of helping to develop methods for protection of line against WIO are a main subject of this work. Original advances studied in this thesis include: - Current state-of-the art for analytical calculation of WIO, including the loads in the wake - Overview of classic theory of wake-induced flutter and its evaluation from the standpoint of modern numerical tools for analytical applications (e.g., Matlab) - Nonlinear Finite-Element Modelling of WIO using classic theory of wake-induced flutter, study of its domains of application, advantages and limitations, including validation upon field experiments - Foundation of basic methodology for optimal placement of spacers over the bundle conductor span
2

Influência da condutividade e permissividade do solo em função da frequência no cálculo da impedância longitudinal e admitância transversal de linhas aéreas de transmissão / Influence of earth's conductivity and permittivity frequency dependence on longitudinal impedance and transversal admittance transmission line's parameters calculation

Gertrudes, João Bosco 16 August 2018 (has links)
Orientador: Maria Cristina Dias Tavares / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação / Made available in DSpace on 2018-08-16T03:57:35Z (GMT). No. of bitstreams: 1 Gertrudes_JoaoBosco_D.pdf: 5500576 bytes, checksum: ef85206048f845d1bff89f7c8948d288 (MD5) Previous issue date: 2010 / Resumo: Esta tese tem como objetivo o desenvolvimento de modelos mais precisos para o cálculo de parâmetros elétricos de linhas aéreas de transmissão para estudos de transitórios na faixa de frequência 0 a 2 MHz. Esta faixa de frequência cobre a maioria das perturbações eletromagnéticas nos sistemas elétricos: energização, faltas simétricas e assimétricas, rejeições de carga, descargas atmosféricas, perturbações harmônicas, etc. Na faixa de frequência citada, em consequência da dependência dos parâmetros do solo com a frequência, a condutividade elétrica do solo (sg) tem a mesma ordem de grandeza que o produto entre a frequência angular do sinal incidente no solo (w) e a permissividade elétrica do solo (eg). Portanto, as suposições de baixa frequência tradicionalmente adotadas - sg constante e weg desprezível (sg >> weg) - podem levar a modelos que não representam de forma adequada a resposta da linha no caso de transitórios rápidos (com espectro de frequência acima de 1 kHz). O objetivo central desta tese é verificar a influência da dependência dos parâmetros do solo no cálculo da impedância longitudinal e admitância transversal por unidade de comprimento em comparação com os modelos tradicionais de cálculo. As análises são feitas inicialmente para o caso de um único condutor acima do solo e posteriormente através de um estudo para uma linha de transmissão trifásica, 440 kV, em operação no sistema elétrico brasileiro. As impedâncias longitudinais e as admitâncias transversais de retorno pelo solo são avaliadas através de métodos tradicionais de cálculo e através de integração numérica das formulações de Carson modificadas para inclusão do modelo de solo que considera a depêndencia de sg e weg em função da frequência / Abstract: This thesis aims to propose more accurate models for calculating the electrical parameters of overhead power transmission lines. The influence of earth's conductivity and permittivity frequency dependence is evaluated when calculating transversal and longitudinal transmission lines' parameters in the frequency range 0 to 2 MHz. This frequency range covers the majority of the electromagnetic transients in electrical systems (switching transients, faults' transients, load rejections, harmonic disturbances, etc). Between 1 kHz to 2 MHz, the product of the signal angular frequency (w) by the dielectric constant (eg) of the soil may have the same order of magnitude as the conductivity (sg), due to the variation of these parameters with frequency. Therefore, the assumptions of low frequency traditionally used - the soil conductivity (sg) considered as constant and weg that can be negligible (sg >> weg) - can lead to incorrect models that do not adequately represent the transmission line's response, in cases of fast transients phenomena (with frequency spectrum above 1 kHz). The importance of properly considering the frequency dependent soil model is presented for the one-conductor case and for a single three-phase transmission line (440 kV, considered ideally transposed). The aim is to compare the transmission line parameters calculated considering the conductivity and weg frequency dependence ground model in relation to the common ground representation, with constant conductivity and weg that may be neglected. It is also compared the results obtained from the complex plane method with those obtained from the Carson's modified expressions for longitudinal parameters and between Carson's modified potential-coefficient correction factors and from the most common approximated methods for transversal admittance parameters / Doutorado / Energia Eletrica / Doutor em Engenharia Elétrica
3

Otimização de linhas de transmissão para manobra de abertura monopolar - análise da influência dos parâmetros transversais do sistema de compensação reativa / Optimization of transmission lines to single-phase opening maneuver - analysis of the influence of transversal parameters of shunt reactive compensation system

Zevallos Alcahuaman, Milton Elvis, 1974- 03 January 2013 (has links)
Orientador: Maria Cristina Dias Tavares / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação / Made available in DSpace on 2018-08-25T04:28:30Z (GMT). No. of bitstreams: 1 ZevallosAlcahuaman_MiltonElvis_D.pdf: 2479285 bytes, checksum: 4c1c5f42e8da1a073caee7cb88308479 (MD5) Previous issue date: 2013 / Resumo: Uma solução bastante utilizada para reduzir a corrente de arco secundário nas linhas aéreas de transmissão em extra-alta-tensão após a ocorrência de faltas monofásicas não permanentes é a inserção de um reator de neutro no arranjo dos reatores dos bancos de compensação reativa em derivação da linha. Muitas vezes estes reatores de neutro não são otimizados, de modo que reatores com valores típicos são especificados nos projetos de linhas de transmissão aéreas, baseados simplesmente no seu nível de tensão, mas sem considerar as especificidades das linhas, como as suas características físicas ou o seu comprimento. A especificação inadequada do reator de neutro pode resultar em correntes de arco secundário elevadas, o que, em última análise, pode reduzir a taxa de sucesso da manobra de religamento monopolar. Além disto, o custo do sistema de compensação pode ser maior do que o necessário. A tese apresenta uma extensa análise de sensibilidade dos principais parâmetros que influenciam no dimensionamento de um tronco de transmissão longo, especificamente do nível de compensação mais adequado e do valor ótimo do reator de neutro. Destaca-se na pesquisa o desenvolvimento de uma representação dos componentes de um sistema de transmissão através de quadripolos trifásicos com o intuito de obter a resposta sustentada durante o desequilíbrio. Através desta representação foi possível identificar a relação entre o reator de neutro ótimo e os parâmetros transversais da linha de transmissão, assim como a influência direta entre o nível de compensação do sistema com o valor do reator de neutro otimizado. Adicionalmente foram identificados comprimentos limites para linhas não compensadas de diferentes níveis de tensão de modo a ainda apresentar alta probabilidade de sucesso para a manobra monopolar. Simulações de faltas monofásicas foram realizadas utilizando dados provenientes de linhas típicas do sistema elétrico brasileiro / Abstract: A widely used solution to reduce secondary arc current in the overhead transmission lines in EHV after single-phase non permanents faults is the insertion of a neutral reactor in the arrangement forming part of the shunt reactive compensation of the line. Frequently in Brazilian electrical system, neutral reactors are not optimized, so that reactors with typical values are specified in the project of overhead transmission lines based on their voltage level, but without considering the specifics line parameters, as their physical characteristics or their length. The specification of unsuitable neutral reactor can result in high secondary arc currents, which eventually diminishes the success rate of single-phase automatic reclosure (SPAR) maneuver. Moreover, the cost of the compensation system may be larger than necessary. This paper presents an extensive sensitivity analysis of the main parameters that influence the project of a long transmission trunk, specifically the most appropriate level of compensation and the optimal value of the neutral reactor. It is noteworthy the development of a transmission system components representation using three-phase two-ports networks in order to obtain sustained response during unbalance. Through this analysis it was possible to identify the relationship between the shunt reactor and the optimized neutral parameter and the influence of the compensation level with the neutral reactor optimized value. Additionally the maximum lengths of non-compensated lines that would have high probability of success of SPAR maneuver were identified. Simulations of single-phase faults were performed using data from typical lines of the Brazilian electric system / Doutorado / Energia Eletrica / Doutor em Engenharia Elétrica

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