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53 Física - Physics

Abstract: lightning induced voltages are one of the most common sources of failures on distribution networks operating in high lightning activity regions. Traditionally, the selection of insulation levels and protecting devices are carried out using statistical analysis based on typical values of resistivity and assuming a homogeneous ground for the whole network. In calculating lightning induced voltages, the effect of the topography and non-homogeneities of the ground have been traditionally neglected. In rural distribution lines, non-homogeneous and non-uniform ground is a common feature. In literature, induced voltages calculations are mainly calculated based on several assumptions that are not valid when more realistic conditions are taken into account. In order to allow a better selection of protective devices and hence contributing to the improvement of some power quality indicators of rural distribution networks, the calculation of lightning induced voltages for distribution lines must be performed including the effects of the non-homogeneous and nonuniform ground. Most of the theoretical approaches proposed for calculating the propagation path effects on the radiated electromagnetic fields for a current dipole above ground, are valid only in the far-field region even when considering irregular and inhomogeneous terrain. Despite some authors have demonstrated the validity of those approaches for flat ground in the near field range calculations, there are valid for some specific cases and geometric symmetry that in some practical cases cannot be assumed. In order to overcome this problem, this thesis presents an extensive application of a full wave solution obtained from the implementation of the Finite Difference Time Domain FDTD method including a non-regular mesh. This method is applied to the calculation of lightning induced voltages on an overhead single wire when different ground features such as: homogeneity, inhomogeneity and non-uniformity are present all simultaneously in a simulation scenario. In order to validate the FDTD implementation, some numerical comparisons were made with previous results presented in the literature. The aim of this thesis is to provide new elements related to the effects on lighting induced voltages on overhead lines when different electric and geometric parameters of the surrounding ground are considered. Along this thesis, the lightning induced voltage problem has been analyzed taking into account three involved aspects individually: the return-stroke model, the propagation of the electromagnetic field produced by it, and the resulting induced voltages on the overhead lines once all their models are included into an FDTD simulation. This document has been divided into eight sections. The first section presents a discussion about lightning induced voltages and how they have been addressed in the literature. Throughout this iii section all the involved elements into the lighting induced problem have been addressed and a short discussion about their previous results and conclusions is also presented. In section 2 the scope of the thesis is defined in order to give the reader a brief summary about the objectives that were established in the master thesis proposal. Section 3 presents the FDTD method. In this section most of the theoretical background is presented related to: sources, lumped elements and thin-wire modeling techniques. Next, the FDTD method is formulated for a non-regular mesh and a general formulation for an automatic meshing algorithm is proposed. Finally, a comparison between the FDTD method implementation used in this thesis and some experimental data from a two horizontal wires cross-talk problem is presented. Section 4 deals with the calculation of radiated fields when different propagation paths are present. Homogeneous ground effects on radiated fields were obtained by using the Norton’s approach and the surface impedance concept. Inhomogeneities of the ground conductivity for flat grounds were also analyzed by using the surface impedance concept and the Waits formula derived from the compensation theorem; the Waits formulas for a mixed-path of two and three section were implemented and compared with some results presented before in literature. Finally,the terrain non-uniformity was addressed by means of the Ott’s integral approach. Despite all of these implemented approaches allow the analysis of radiated fields, they are derived under several assumptions and are valid only for the far field region and a cylindrical symmetry regarding geometry. Then, a comparison between these and the results obtained by means of the FDTD method were performed for different simulation scenarios in order to analyze their validity. In section 5 the lightning return-stroke is modeled by means of an implementation of engineering and electromagnetic models. A discussion about the current distribution along the cannel depending on the return-stroke model is also presented. Besides, a comparison between the antenna theory and the series RL-loaded thin-wire model included into the FDTD method was carried out taking into account the characteristics of apparent propagation velocity and current wave shape along the channel. In section 6 the lightning radiated fields are calculated for different propagation path conditions such as: perfectly conducting ground, homogeneous finitely conductive ground and inhomogeneous conducting ground. For those propagation paths a set of comparisons between the FDTD method and the approximated formulas discussed in section 5 were performed. Lightning induced voltages are analyzed in section 7. In this section the lightning channel and the overhead line are included into the FDTD method. A set of simulations scenarios were proposed in order to evaluate the influence of different ground features on the induced voltages on a single overhead-wire. Important influences on induced voltage waveforms were determined for inhomogeneous and irregular terrains, resulting in changes on polarity and higher induced peak voltages values when compared to those obtained from a flat homogeneous ground. iv In section 8 concluding remarks about the analyzed cases and most critical situations are presented. There is also a future work proposed by the author based on the obtained results

Tipo de documento: Tesis-trabajos de grado - Thesis Maestría

Colaborador - Asesor: Herrera Murcia, Javier Gustavo

Información adicional: Maestría en Ingeniería Eléctrica

Palabras clave: Lightning return strokes, Lightning induced voltages, Finite-Difference Time-Domain FDTD, Radiowave propagation over ground, Fenómenos transitorios Electricidad, Estabilidad de sistemas de energía eléctrica, Rayos atmosféricos, Transients Electricity, Electric power system stability, Lightning

Temática: 5 Ciencias naturales y matemáticas - Science 53 Física - Physics6 Tecnología ciencias aplicadas - Technology 62 Ingeniería y operaciones afines - Engineering





Fuente: http://www.bdigital.unal.edu.co


Introducción



Lightning Induced Voltages on Overhead Lines above Non-Uniform and NonHomogeneous Ground Author Raúl Esteban Jiménez Mejía Master Thesis Dissertation Universidad Nacional de Colombia Departamento de Energía Eléctrica y Automática Facultad de Minas November 2014 Lightning Induced Voltages on Overhead Lines above Non-Uniform and NonHomogeneous Ground Author Raúl Esteban Jiménez Mejía Master Thesis Dissertation Presented as a partial fulfillment of the requirements for the degree of Master on Electrical Engineering Advisor Prof.
Javier Gustavo Herrera Murcia, Ph.D. Universidad Nacional de Colombia Departamento de Energía Eléctrica y Automática Facultad de Minas November 2014 Gratefulness: To my Parents Acknowledgments There are many people that I must be grateful with by all of their attention and comprehension along these two years.
I would like to express my sincere gratitude to all of those that have been there offering me their valuable comments and assistance during the development of this thesis. Along the time working on it, I have learned not only about lighting induced voltages but also that when you are with a good working group and excellent people around you, there are more probabilities to succeed. I wish to thank Prof.
Javier Herrera for accepting to be the advisor of this work and the way how he from the first time offered me all his experience on lightning research for the development of this thesis. I really have to highlight all the support that I have received from the Research Group on Applied Technologies (GITA) and express my especial gratitude to Prof.
Guillermo Mesa who has been a very important advisor not only academically but also as a real and sincere friend.
I would also like to thank to Prof.
Clara Rojo for all of her valuable advices and her support whereas I held my position as teaching assistant of the high voltage laboratory. My sincere gratitude to my parents for all of their confidence on me and on my work.
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