The Lightning Flash
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This unique book provides the reader with a thorough background in almost every aspect of lightning and its impact on electrical and electronic equipment. The contents range from basic discharge processes in air throughtransient electromagnetic field generation and interaction with overhead lines and underground cables, to lightning protection and testing techniques.
Inspec keywords: electromagnetic compatibility; lightning protection; power overhead lines; lightning; electric fields; discharges (electric); thunderstorms
Other keywords: thunderclouds; electrical discharge; electric radiation field; overhead electrical networks; lightning strike; electromagnetic field computation; lightning discharge; lightning protection; mathematical modeling; EMC; lightning flash; charge structure; thunderstorm electrification mechanism; geographical variation; return stroke
Subjects: Atmospheric electricity; Dielectric materials and properties; Power system protection; Overhead power lines; Electromagnetic compatibility and interference
- Book DOI: 10.1049/PBPO034E
- Chapter DOI: 10.1049/PBPO034E
- ISBN: 9780852967805
- e-ISBN: 9781849190497
- Page count: 598
- Format: PDF
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Front Matter
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1 Charge structure and geographical variation of thunderclouds
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This chapter discusses the formation of clouds and the local conditions necessary for the formation of thunderclouds. The gross charge structure and geographical variability of thunderclouds are also presented. Discussions on sprite-producing thunderclouds and mesoscale convective systems are also included.
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2 Thunderstorm electrification mechanisms
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The origin of thunderstorm electrification has long been an unsolved problem in atmospheric physics. Despite a number of simulated laboratory experiments, together with the vast amount of field data collected over the past few decades, our knowledge of how these convective cloud masses get charged still remains sparse at the microphysical level. Sir John Mason in the Bakerian Lecture identified thunderstorm electrification as one of the three leading unsolved problems in cloud physics. He had this to say about the problem: This is, for me, the most intriguing and challenging problem in cloud physics, with a strong incentive to understand one of the most spectacular of natural phenomena, but made all the more interesting by the fact that the search for a continuing solution has led us into a number of rather difficult areas of classical physics, and to a deeper study of the fundamental properties of water and ice. A satisfactory theory must be able to explain all of the observed electrical characteristics of a typical thunderstorm.
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3 Mechanism of electrical discharges
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The experiments performed by researchers in different countries, notably, South Africa, England, Switzerland and the USA, during the last sixty years have greatly advanced our knowledge concerning the mechanism of lightning flashes. However, many pieces of the puzzle are still missing and many of the theories put forth as explanation of its mechanism are mainly of a qualitative nature. The reason for this slow progress is the impossibility of studying lightning flashes under controlled lab oratory conditions. On the other hand, the mechanism of the electric spark, which could be studied under controlled conditions, may guide the researchers in their quest for understanding the mechanism of lightning flashes and creating more advanced theories of the phenomena. After all, it is the observed similarities between the small laboratory sparks and lightning discharges that forced Benjamin Franklin to conclude that the lightning flash is a manifestation of electricity. This chapter is devoted to a description of the mechanism of laboratory sparks.
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4 The mechanism of the lightning flash
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Experimental observations of the optical and electromagnetic fields generated by lightning flashes during the last 50 years have significantly advanced our knowledge concerning the mechanism of the lightning flash. Nevertheless, this knowledge is not as exhaustive as that of long laboratory sparks due to our inability to observe lightning flashes under controlled conditions. Thus, the mathematical description of the mechanism of a lightning flash is relatively poor at present even though the main features of lightning flashes themselves are well known. The main goal of this chapter is to provide the reader with the important features of the mechanism of the lightning flash. No attempt is made to provide an exhaustive list of the literature since this can be found elsewhere. Nomenclature: in this chapter a positive discharge is defined in such a way that the direction of motion of electrons in such a discharge is opposite to that of the discharge itself; a negative discharge is defined as one in the opposite sense. According to this definition a negative return stroke is a positive discharge and a positive return stroke is a negative discharge. A positive field is defined in terms of a negative charge being lowered to ground or positive charge being raised. According to this definition a lightning flash that transports negative charge to ground gives rise to a positive field change.
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5 Computation of electromagnetic fields from lightning discharge
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Electromagnetic fields from lightning can couple to electrical systems and produce transient overvoltages, which can cause power and telecommunication outages and destruction of electronics. Therefore calculation of the electric and magnetic fields from different lightning processes has practical applications. In this chapter expres sions for electric and magnetic fields are derived for some simplified charge and current configurations applicable to lightning. In general, lightning currents and charges vary with time. First, simple expressions for nontime-varying cases are presented, then electric and magnetic field expressions from time-varying lightning sources are given.
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6 Mathematical modelling of return strokes
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A lightning flash is initiated by electrical breakdown of the air in a cloud. This process, commonly known as the preliminary breakdown, signifies the initiation of a stepped leader. Such a stepped leader propagates towards the earth, in a succession of nearly discontinuous surges or steps. The stepped leader leaves a charged, conducting channel in its wake. When the leader reaches the ground, the current flowing in the channel increases abruptly, marking the beginning of the return stroke. After the first return stroke, several subsequent return strokes may occur, each of which is preceded by a fast, continuously moving leader the dart leader which propagates from cloud to earth down the channel made by the stepped leader. This chapter is concerned with the mathematical modelling of return strokes.
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7 The effects of propagation on electric radiation fields
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The protection of structures and electrical systems from lightning requires knowledge of the characteristics of electromagnetic fields generated by lightning and of the statistical distribution of lightning current parameters. The statistical distributions of lightning current parameters can be obtained by recording the currents in lightning flashes striking high tower. However, the presence of the tower itself may distort these distributions to some extent and there is always the unresolved question of whether these distributions are valid for lightning flashes striking flat regions. On the other hand, all the information necessary to obtain the characteristics of currents in lightning return strokes is embedded in the lightning-generated electromagnetic fields. However, in propagating from source to measuring station, the electromagnetic fields will change in a number of ways depending on the geometry and the electrical characteristics of the propagation path. For example, in propagating over finitely conducting ground, the electromagnetic fields will lose their higher frequency components. As a result, the amplitude of the electromagnetic field decreases, and the rise time of the electromagnetic field increases with increasing propagation distance over land. When the ground is stratified the propagation effects may enhance or attenuate high frequencies, depending on the conductivity and the depth of the conducting layers.
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8 Interaction of electromagnetic fields generated by lightning with overhead electrical networks
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In this chapter, we will present the theory describing the interaction of lightning electromagnetic fields with overhead lines, with particular reference to power systems. In the first part of the chapter, we will present the different approaches and formulations that can be used to describe the coupling between an external electromagnetic field and a transmission line. Then, we will extend the selected field-to-transmission line coupling model to include the effects of a lossy earth serving as a return conductor and to deal with the case of multiconductor lines. The time-domain representation of coupling equations, useful for analysing nonlinearities, will also be dealt with.
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9 Lightning and EMC
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In order to understand why lightning has again become a major item of concern for many applications of electricity a short look into the history of electromagnetic compatibility (EMC) is necessary.
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10 Principles of protection of structures against lightning
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Thunderstorms are natural weather phenomena and there are no devices and methods capable of preventing lightning discharges. Direct and nearby cloud-to-ground discharges can be hazardous to structures, persons, installations and other things in or on them, so that the application of lightning protection measures must be considered.
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11 Electrical aspects of lightning strike to humans
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In this segment models for in-the-field strike and telephone-mediated strike have been developed, and proposals regarding pathways have been made. An estimation of the magnitude and the time course of the insult has been given.
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Back Matter
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