Experimental and numerical analysis of cellulosic insulation failures of continuously transposed conductors under short circuits and thermal ageing in power transformers

The integrity of the cellulosic insulation in power transformers is considered one of the most relevant parameters that affects their performance and reliability. Electric faults, such as short circuits, have thermal and mechanical effects that degrade the paper and can eventually produce the end-of-life of the transformer. The evolution of the properties of the paper insulation of a commercial continuously transposed conductor due to thermal ageing was characterised through the degree of polymerisation and tensile testing. Failure initiation and propagation in the paper was analysed macroscopically and microscopically using scanning electron microscope. A finite element numerical mechanical model of the conductor was implemented to reproduce the experiments and to obtain the load level and strain state that produce failure at each ageing state, aiming at developing a failure model for the insulation. This model may contribute to an improvement in manufacturing processes and management of the electrical system

Estudio experimental, analítico y numérico del fallo mecánico del aislamiento celulósico de conductores continuamente transpuestos en transformadores de potencia bajo la influencia de cortocircuitos y envejecimiento térmico

RESUMEN: El papel dieléctrico de un transformador de potencia está sometido a un proceso de envejecimiento, producido por altas temperaturas y reacciones químicas en el aceite dieléctrico. Las fuerzas electromagnéticas que afectan a los conductores pueden producir la rotura del papel, llegando a causar el fin de la vida útil del transformador. Para mejorar la comprensión de estos fenómenos, un conductor continuamente transpuesto (CTC) con cuatro capas de dieléctrico (papel Kraft ordinario y tipo crepe) fue impregnado en aceite nafténico y envejecido durante diferentes periodos, representando toda su vida útil. Muestras del CTC fueron sometidas a deformaciones de flexión compatibles con un cortocircuito, analizándose las grietas generadas en el papel. El papel dieléctrico envejecido fue caracterizado mecánicamente (a tracción, compresión y tangencialmente), y el ensayo a flexión se simuló numéricamente en ANSYS Static Structural, para estimar las deformaciones en el papel. Dos criterios analíticos de fallo (máxima deformación y criterio de Norris) fueron adaptados para predecir las regiones del papel dieléctrico susceptibles de fallar para un estado de envejecimiento y deformación impuesta determinados, obteniendo predicciones coherentes con los resultados experimentales y validando así el enfoque de Norris.ABSTRACT: The dielectric paper of power transformers is subjected to ageing caused by high temperatures and chemical reactions in the dielectric oil. The electromagnetic forces the conductor is subjected to can cause the breakage of the paper and even the end of the useful life of the transformer. To improve the understanding of those phenomena, a continuously transposed conductor (CTC) with four layers of dielectric paper (plain Kraft and crepe papers) was impregnated in naphthenic oil and aged during different periods, representative of its whole lifespan. CTC samples were subjected to bending deformations compatible with a short circuit, and the cracks generated in the paper were analysed. The aged paper was mechanically characterised (in tensile, compressive and shear modes) and the bending tests were simulated in ANSYS Static Structural, to estimate the strains in the paper. Two analytical failure models (maximum strain and Norris criterion) were modified to predict the regions in the dielectric paper susceptible to fail for a particular ageing state and deformation level, obtaining a good agreement with the experimental results and thus validating the Norris approach

Vibrational energy harvesting for sensors in vehicles

The miniaturization of semiconductor technology and reduction in power requirements have begun to enable wireless self-sufficient devices, powered by ambient energy. To date the primary application lies in generating and transmitting sensory data. The number of sensors and their applications in automotive vehicles has grown drastically in the last decade, a trend that seems to continue still. Wireless self-powered sensors can facilitate current sensor systems by removing the need for cabling and may enable additional applications. These systems have the potential to provide new avenues of optimization in safety and performance.This thesis delves into the topic of vibrations as ambient energy source, primarily for sensors in automotive vehicles. The transduction of small amounts of vibrational, or kinetic, energy to electrical power, also known as vibrational energy harvesting, is an extensive field of research with a plethora of inventions. A short review is given for energy harvesters, in an automotive context, utilizing transduction through either the piezoelectric effect or magnetic induction. Two practical examples, for ambient vibration harvesting in vehicles, are described in more detail. The first is a piezoelectric beam for powering a strain sensor on the engines rotating flexplate. It makes combined use of centrifugal force, gravitational pull and random vibrations to enhance performance and reduce required system size. The simulated power output is 370 \ub5W at a rotation frequency of 10.5 Hz, with a bandwidth of 2.44 Hz. The second example is an energy harvesting unit placed on a belt buckle. It implements magnetic induction by the novel concept of a spring balance air gap of a magnetic circuit, to efficiently harvest minute vibrations. Simulations show the potential to achieve 52 \ub5W under normal road conditions driving at 70 km/h. Theoretical modeling of these systems is also addressed. Fundamental descriptions of the lumped and distributed models are given. Based on the lumped models of the piezoelectric energy harvester (PEH) and the electromagnetic energy harvester (EMEH), a unified model is described and analyzed. New insights are gained regarding the pros and cons of the two types of energy harvester run at either resonance or anti-resonance. A numerical solution is given for the exact boundary of dimensionless quality factor and dimensionless intrinsic resistance, at which the system begins to exhibit anti-resonance. Regarding the maximum achievable power, the typical PEH is favored when running the system in anti-resonance and the typical EMEH is favored at resonance. The described modeling considers all parameters of the lumped model and thus provides a useful tool for developing vibrational energy harvester prototypes

Index to 1981 NASA Tech Briefs, volume 6, numbers 1-4

Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1981 Tech Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

Structural integrity of power transformers

The subject of this thesis is to study the mechanical integrity and performance of insulation materials used in power transformers under short circuit conditions. There are a number of methods to calculate short circuit forces in the literature. These methods were developed to determine the magnetic properties related to the short circuit condition and cannot be used in the open circuit condition due to the assumption of infinitely permeable core. To this end, a new solution strategy is introduced which is able to calculate magnetic properties of power transformers both in open circuit and in short circuit conditions. A solution was derived utilizing transform techniques and multiple connected permeable regions can now be solved. Mechanical failure modes of transformer winding are presented and new solution methods are introduced for some failure modes. Dynamic representation of the transformer winding is achieved by treating the winding turn by turn. Particular attention is given to stress calculations of Continuously Transposed Cable (CTC) and resin bonded CTC. Digital Image Correlation (DIC) technique maps the strain distribution on the test specimen and the strain distribution can be extracted at any cross section of interest. 3‐D DIC technique is used to determine the response of transformerboard material under tensional and bending loads and material properties are determined. It is also shown that the DIC method provides much more accurate results compared to strain gauges due to its manufacturing technique. DIC results are used to determine material properties related to both tensional and bending type loading conditions. Resin laminate wood is also compared to transformerboard. Viscoelastic properties of transformerboard are examined. Because a short circuit event is dynamic in nature, storage and loss modulus of transformerboard are determined as a function of temperature and frequency under cyclic loading conditions. Conventional creep test setups cannot be used for transformerboard. A test setup is designed to measure creep curves of transformerboard. The test setup measures the creep curves both in oil and in air and the displacements are recorded automatically. Tests are performed at a wide range of temperatures encountered in real service conditions of transformerboard material. The developed method can be introduced in PC codes to determine the magnetic properties related to the magnetic field. Also the method can be used to determine turn to turn or disc to disc mutual inductances of a transformer. Electromagnetic forces calculated with the introduced method are also more accurate than the methods developed earlier. Methods to calculate mechanical stresses acting on transformer components are also reviewed and new solution techniques for some failure modes are developed. Mechanical performance of insulation materials under tension and bending type loadings are measured and monitored using DIC technique. Finally, time dependent material properties of the transformerboard under constant and cyclic loading conditions are determined. Both analytical and experimental techniques are utilized to determine the material properties of the transformer components and their behaviour under different loading conditions. New solution techniques are developed and the material behaviour of the insulation materials under different loading conditions is determined. With the data obtained and solutions developed, mechanical stress calculations of the power transformer components can be made more accurately

Characterization of Power Transformer Frequency Response Signature using Finite Element Analysis

Power transformers are a vital link in electrical transmission and distribution networks. Monitoring and diagnostic techniques are essential to decrease maintenance and improve the reliability of the equipment.This research has developed a novel, versatile, reliable and robust technique for modelling high frequency power transformers. The purpose of this modelling is to enable engineers to conduct sensitivity analyses of FRA in the course of evaluating mechanical defects of power transformer windings. The importance of this new development is that it can be applied successfully to industry transformers of real geometries

Topological optimization of structures produced through 3D printing of fiber reinforced cementitious materials

Dissertação de mestrado integrado em Engenharia CivilTopology optimization can play an important role in the Architecture, Engineering and Construction (AEC) sector. This technology along with digital manufacturing can be a game changer in the future of civil construction, allowing to build, in a short time period, lighter constructions with very geometry complexity but keeping the same of even better structural functioning. These optimized structures when coupled with a material with high capacity efforts redistribution, e.g. fibre reinforced cementitious material (FRC), can partially or totally substitute the conventional reinforcement, consequently less raw material is use, contributing for a better sustainable development. Following this idea, this dissertation will focus on study topology optimization processes along with the use of FRC materials. Initially a comparison between some topology optimization software’s will be carried out, in order to proper evaluate to most suitable for the realization of the present work. In a second stage, considering only the linear behavior of the material, different topology optimization analyses will be done. These analyses will be based on the geometry and the intended structural application (support and load conditions), in addition to the optimization goal (design variable and constraint). This part aims to assess the influence of height / length ratio (H/L ratio) of the beam, in the optimization outcome. After that, a study of the influence of reinforcement amount in the optimization will be done. Afterwards, some finite element analysis (FEA) for one of the optimized structures will be performed and assessed using distinct approaches for obtaining the tensile stress – strain relationship, namely by adopting the ultimate limit state (USL) and service limit state (SLS) tensile diagrams according to the recommendations presented in FIB Model Code 2010. These simulations will serve to evaluate the nonlinear behavior of the FRC structure. For this study six FRC with different strength classes were considered. Finally, an optimized structural element obtained through the FEA was sliced for 3D printing and the influence of the nozzle dimensions, i.e. printing resolution was checked.A otimização da topologia pode desempenhar um papel importante no setor de Arquitetura, Engenharia e Construção (AEC). Esta tecnologia aliada à manufatura digital pode completamente revolucionar o futuro da construção civil, permitindo construir, num curto espaço de tempo, construções mais leves, mas mantendo o mesmo ou ainda melhor funcionamento estrutural. Estas estruturas otimizadas quando conjugadas a um material com alta capacidade de redistribuição de esforços, por ex. materiais cimentícios reforçado com fibras (FRC), pode substituir parcial ou totalmente o reforço convencional, onde consequentemente menos matéria-prima será utilizada, contribuindo-se assim, para um melhor desenvolvimento sustentável. Seguindo essa ideia, esta dissertação terá como foco estudar processos de otimização de topológica juntamente com o uso de materiais FRC. Inicialmente será realizada uma comparação entre alguns softwares de otimização de topológica, a fim de avaliar adequadamente o mais adequado para a realização do presente trabalho. Em uma segunda etapa, considerando apenas o comportamento linear do material, serão realizados diferentes processos de otimização topológica. Essas otimizações serão baseadas na geometria e na aplicação estrutural pretendida e no objetivo da otimização. Esta parte visa avaliar a influencia da relação altura/comprimento da viga (relação H/L), no resultado da otimização. Posteriormente, algumas análises de elementos finitos (FEM) para uma das estruturas otimizadas serão realizadas e avaliadas usando duas abordagens distintas para a obtenção da relação tensão de tração – deformação, uma para estado limite último (ELU) e estado limite de serviço (ELS), seguindo as recomendações presentes no FIB Model Code 2010. Estas simulações servirão para avaliar o comportamento não linear da estrutura de FRC. Para este estudo foram considerados seis FRC com diferentes classes de força. Finalmente, para um elemento estrutural otimizado anteriormente, foi realizada uma simulação de impressão 3D, de modo a estudar a influencia do tamanho do bico de impressão, ou seja, a resolução de impressão foi verificada

Advances in Vibration Analysis Research

Vibrations are extremely important in all areas of human activities, for all sciences, technologies and industrial applications. Sometimes these Vibrations are useful but other times they are undesirable. In any case, understanding and analysis of vibrations are crucial. This book reports on the state of the art research and development findings on this very broad matter through 22 original and innovative research studies exhibiting various investigation directions. The present book is a result of contributions of experts from international scientific community working in different aspects of vibration analysis. The text is addressed not only to researchers, but also to professional engineers, students and other experts in a variety of disciplines, both academic and industrial seeking to gain a better understanding of what has been done in the field recently, and what kind of open problems are in this area