1,721,470 research outputs found
Modeling lightning strikes to tall towers
No full textIn this chapter, we present a review of recent progress in the modeling of lightning strikes to tall structures. Since some tall structures are struck by lightning several tens of times per year, they can be used as ground-truth to measure and calibrate the location accuracy of lightning location systems. In addition, knowledge of the transient processes in tall objects when they a subjected to a lightning strike allows us to use them to calibrate the lightning return stroke currents reported by lightning detection and location systems. Tall objects constitute also a primary source of data from which channel-base lightning current statistics are obtained. These statistics are in turn used to improve the design of lightning protection devices and systems. This chapter is organized as follows: Section 5.2 presents a review of the extension of lightning return stroke models to include the presence of an elevated strike object. Section 5.3 deals with the computational methods for the evaluation of the electromagnetic fields generated by lightning strikes to tall structures. A review of available data on lightning currents from lightning to tall structures is presented in Section 5.4. Finally, a summary is given in Section 5.5.SCI-STI-F
Interaction of lightning-generated electromagnetic fields with overhead and underground cables
No full textIn this chapter, we discussed the TL theory and its application to the problem of lightning electromagnetic field coupling to transmission lines. After a short discussion on the underlying assumptions of the TL theory, we described seemingly different but completely equivalent approaches that have been proposed to describe the coupling of electromagnetic fields to transmission lines. The field-to-transmission line coupling equations were then extended to deal with the presence of losses and multiple conductors and expressions for the line parameters, including the ground impedance and admittance were presented. The time-domain representation of the field-to-transmission line coupling equations, which allows for a straightforward treatment of nonlinear phenomena as well as the variation in the line topology, was also described. Solution methods in the frequency domain and in the time domain were given and application examples with reference to lightning-induced voltages were presented and discussed. Specifically, the effect of ground losses was illustrated and discussed. When the travelling voltage and current waves are originated from lumped excitation sources located at a specific location along a transmission line (direct lightning strike), both the corona phenomenon and ground losses result in an attenuation and dispersion of propagating surges along transmission lines. However, when distributed sources representing the action of the electromagnetic field from a nearby lightning illuminating the line are present, ground losses and the corona phenomenon could result in important enhancement of the induced voltage magnitude. Finally, we reviewed the theory of electromagnetic field coupling to a buried cable. Solution methods in the frequency and the time domain were also presented. Examples of lightning-induced currents and comparison with experimental data were presented.SCI-STI-F
Lightning interaction with the ionosphere
No full textLightning discharges, including cloud-to-ground (CG) and intracloud (IC) lightning, are known to emit electromagnetic pulses (EMPs) in a wide frequency band ranging from few Hz up to hundreds MHz [1]. During the breakdown and ionization processes (mostly from leader processes and streamers), there are strong emissions in the HF (3-30 MHz) and VHF (30-300 MHz) bands. When high currents occur in previously ionized channels (mostly from return strokes and the active stage of cloud flashes), the most powerful emissions concentrate in the very low frequency (3-30 kHz, VLF) and low frequency (30-300 kHz, LF) bands [2]. Among them, the VLF/LF waves of lightning discharges can propagate long distances with low attenuation by reflection between the ground surface and the lower D-region ionosphere (60-90 km), namely the so-called earth-ionosphere waveguide (EIWG). In order to investigate the lightning EMPs interaction with the ionosphere, a number of models and methods have been developed in the literature, such as the wave-hop (ray theory) method [3-6], the waveguide mode theory [7-9], or numerical methods such as the finite-difference-time-domain (FDTD) method [10-17] and the full-wave finite element method (FEM) [18,19]. Previous studies indicate that the amplitude and phase perturbation for lightning VLF/LF signals have a complicated relationship with the ionospheric D region parameters. The propagation of lightning EMPs between the earth ground surface and the lower D region ionosphere can be affected by many factors, such as the propagation distances [10,14,20], the ground conductivity [14,20], the electron and neutral particle densities [13,21,22], the Earth curvature [23,24], the presence of the Earth's magnetic field [22,25-27], and the presence of mountainous terrain [24]. In this chapter, we will first introduce the propagation theory of lightning EMPs interaction with the ionosphere on the basis of the full-wave FDTD method. We will then investigate the propagation effect of lightning radiated electromagnetic (EM) fields in the EIWG by considering the effect of the Earth curvature, the effect of the ground conductivity, and the effect of different ionospheric profiles. Finally, we will present applications, including (1) propagation of narrow bipolar events (NBEs) at different distances, (2) lightning electromagnetic fields propagation over mountainous terrain, and (3) the optical emissions of lightning-induced transient luminous events in the nonlinear D-region ionosphere.SCI-STI-F
Full-wave analysis of wind turbine transient response to direct lightning strikes
No full textLightning strikes attachment to Wind Turbines (WTs) is not uncommon. The current discharge along the WT components can cause variations in the electromagnetic field radiation, affecting the surrounding power and electronic systems. In this contribution, a WT transient response to direct lightning strikes is studied with a full-Maxwell approach, involving a complete model for the WT. Simulations are carried out considering both first and subsequent return strokes to one of the rotor blades. Significant reflections are found in the lightning current waveforms through the WT, and this gives rise to amplifications in the radiated electromagnetic fields with respect to ground discharges
On the Fourier transform of measured electric fields radiated by a lightning return stroke
No full textThe Fourier transform (FT) of the electric field (EF) radiated by a lightning return stroke is shown to include a distributional term at zero frequency, which prevents numerical algorithms, such as fast FT, from working properly. Inspired by the finite duration of any field measurement in real applications, we introduce a time-windowing strategy, whereby the integration time-interval of the FT becomes bounded. As a result, the distributional term is removed from the FT, whereas the functional part of the FT is modified throughout the whole range of frequencies, in such a way that a good performance of the numerical algorithms can be restored. The dependence of the modified spectra of the EFs on the length of the time window is also investigated and three numerical experiments with different channel-base currents are performed to validate the whole resulting framework
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
An equivalent attenuation function for the computation of the lightning field radiated by a tortuous channel
No full textThe electromagnetic lightning field is usually computed by considering a model consisting in a vertical and rectilinear channel, as well as a Transmission-Line (TL) type propagation characterized by an attenuation function, which expresses how the current decreases while it travels upward along the lightning channel. When dealing with realistic channels, affected by tortuosity, the current attenuation is
usually neglected and the field computation is cumbersome, since the discretization of the channel in a large number of linear segments has to be performed. This contribution presents an equivalent attenuation function to be used in the computation of vertical channels in order to reproduce the results of a tortuous one. The attenuation function is computed by solving an inverse problem through the Tikhonov regularization technique, and it is found to be reliable for distances larger than 5 km from the lightning channel
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