D. Pavanello

Far-field-current relationship based on the TL model for lightning return strokes to elevated strike objects

New general expressions relating lightning return stroke currents and far radiated electric and magnetic fields are proposed, taking into account the effect of an elevated strike object, whose presence is included as an extension to the transmission line (TL) model. Specific equations are derived for the case of tall and electrically short objects. The derived expressions show that, for tall structures (when the round-trip propagation time from top to bottom within the tower is greater than the current zero-to-peak risetime), the far field is enhanced through a factor with respect to an ideal return stroke initiated at ground level. The enhancement factor can be expressed in terms of the return stroke wavefront speed v, the speed of light in vacuum c, and the current reflection coefficient at the top of the elevated strike object. For typically negative values of this top reflection coefficient, lightning strikes to tall towers result in a significant enhancement of the far electromagnetic field. Expressions relating the far electromagnetic field and the return stroke current are also presented for electrically short towers and for very long return stroke current wavefronts. For the case of return strokes initiated at ground level (h=0), these expressions represent a generalization of the classical TL model, in which the reflections at the ground are now taken into account. We describe also simultaneous measurements of return stroke current and its associated electric and magnetic fields at two distances related with lightning strikes to the 553-m-high Toronto Canadian National (CN) Tower performed during 2000 and 2001. The derived expressions for tall strike objects are tested versus obtained sets of simultaneously measured currents and fields associated with lightning strikes to the CN Tower, and a reasonable agreement is found. Additionally, it is shown that the peak of the electromagnetic field radiated by a lightning strike to a 553-m-high structure is relatively insensitive to the value of the return stroke velocity, in contrast to the lightning strikes to ground.

On the Current Peak Estimates Provided by Lightning Detection Networks for Lightning Return Strokes to Tall Towers

The peak current estimation of lightning detection networks for strikes to tall towers is discussed in this paper. Such systems are sometimes calibrated using return-stroke current data obtained by means of rocket-triggered lightning or instrumented towers of relatively short height. However, for strikes to electrically tall towers, they tend to overestimate the return-stroke current peak. In this case, in fact, the associated radiated electromagnetic fields, from which the return-stroke current is estimated, experience a significant enhancement with respect to the field that would be radiated if the same return stroke was initiated at ground level or on a short tower. Two approaches to correct the current estimates of a lightning detection network for a lightning strike to a tall tower are discussed and applied to the current measurements obtained at the CN Tower in Toronto in the summer of 2005, for which estimates were available from the North American Lightning Detection Network (NALDN). It is shown that correcting the NALDN estimates using the so-called tower factor obtained from theoretical studies results in an excellent estimation of lightning current peaks.

Lightning strikes to elevated structures: influence grounding conditions on currents and electromagnetic fields

This paper presents an analysis of lightning return strokes to tall structures. The interaction of lightning with a tall structure is modeled using the antenna theory. The finite ground conductivity as well as the buried grounding system of the tall structure are taken into account in the analysis. It is shown that the current waveform, in sections of the tower close to ground, is somewhat affected by a finite ground conductivity. However, for sections further up the tower, it is not significantly influenced. Furthermore, our simulations show that some fine structure associated with current waveforms measured on the Toronto CN tower can be attributed to the finite ground conductivity. It is also shown that the current path down the tower structure is notably subjected to the skin effect. The current distribution along the buried grounding structure of the tower is also presented, illustrating the dispersion effect as a function of the ground conductivity. Finally, the lightning return-stroke generated electric and magnetic fields computed at a distance of 2 km from the tower are presented. It is shown that some late-time subsidiary peaks are smoothed out by the effect of the propagation along a finitely-conducting ground.

Radiated Fields From Lightning Strikes to Tall Structures: Effect of Upward-Connecting Leader and Reflections at the Return Stroke Wavefront

An extension of the engineering return-stroke models for lightning strikes to tall structures that takes into account the presence of possible reflections at the return-stroke wavefront and the presence of an upward-connecting leader is presented. Based on the approach proposed by Shostak et al., closed-form, iterative solutions for the current distribution along the channel, and the strike object are derived. Simulation results for the magnetic fields are compared with experimental waveforms associated with lightning strikes to the CN Tower (553 m). It is shown that taking into account the reflections at the return-stroke wavefront results in better agreement with the fine structure of the magnetic-field waveforms. Moreover, the obtained results are in better agreement with experimental observations, reproducing both the early narrow undershoot and the far-field zero crossing. The results also suggest that the typical double-peak response of the radiated fields from tall structures might be due to the combined effect of upward-connecting leaders and reflections at the return-stroke wavefront.

Lightning electromagnetic fields at very close distances associated with lightning strikes to the Gaisberg tower

In this paper we present and discuss measurements of electric (vertical and radial) and magnetic fields from leaders and return strokes associated with lightning strikes to the 100 m tall Gaisberg tower in Austria obtained in 2007 and 2008. The fields were measured at a distance of about 20 m from the tower. Simultaneously, return stroke currents were also measured at the top of the tower. The data include, for the first time at such close distances, simultaneous records of vertical and horizontal electric fields. The vertical electric field waveforms appeared as asymmetrical V-shaped pulses. The initial, relatively slow, negative electric field change is due to the downward leader, and the following, fast, positive electric field change is due to the upward return stroke phase of the lightning discharge. The horizontal (radial) electric field due to the leader phase has a waveshape similar to that of the vertical electric field. However, the horizontal field due to the return stroke is characterized by a short negative pulse of the order of 1 mu s or so, starting with a fast negative excursion followed by a positive one. The return stroke vertical electric field changes appear to be significantly smaller than similar measurements obtained using triggered lightning. This finding confirms the shadowing effect of the tower, which results in a significant decrease of the electric field at distances of about the height of the tower or less. The vertical and horizontal E field changes due to the return stroke were also found to be larger on average than the leader electric field changes. In a significant number of cases (33%), the vertical electric field waveforms due to the return stroke were characterized by a first peak exceeding the typical late-time flattening due to the electrostatic term. This is in contrast with similar measurements related to triggered lightning which do not exhibit such a first peak. About one quarter of the measured vertical electric field waveforms (18 pulses out of 76) featured an unusual waveform characterized by a positive leader field change followed by a bipolar return stroke field change with a zero crossing time of about 60 mu s.

Measurement of Lightning Currents Using a Combination of Rogowski Coils and B-Dot Sensors

The paper presents laboratory tests and a characterization of commercial Rogowski coils and a specially designed B-dot sensor for the measurement of lightning currents on the Säntis telecommunications tower in Switzerland. In order to overcome the limited high frequency response of the Rogowski coils, we propose to use magnetic loops located very close to the tower. We introduce the design of a B-dot sensor based on improvements proposed by C. E. Baum, the inductance of which is lowered by extending the vertical dimension of the loop, with the overall effect of extending the operating frequency range. To overcome the integrating behavior of the loop when connected to low-impedance loads (such as 50 Ohm), an arrangement of 100-Ohm-impedance cables connected across conical-transmission-line gaps is used. The designed sensor is characterized by an upper frequency cutoff of 20 MHz and a 50 Ohm matched termination. Laboratory tests carried out in the high voltage laboratory of the EPFL show the effectiveness of the simultaneous use of Rogowski coils and B-dot sensors for the measurement of lightning currents.

Effect of Nearby Buildings on Electromagnetic Fields from Lightning

We present a discussion on the effect of nearby buildings on the electric and magnetic fields radiated by lightning. Electric and magnetic fields radiated from distant natural lightning have been measured simultaneously on the roof of a building (the Power Systems Laboratory of the Swiss Federal Institute of Technology, Lausanne, Switzerland) and on the ground at different distances away from it. The results suggest that the measured electric field on the roof of the 9-m tall building is enhanced by a factor of 1.7 to 1.9, whereas the electric fields on the ground experience a significant reduction due to the shadowing effect of the building. Also, it is shown that for a sensor located on the ground, close to a building, the magnetic field component perpendicular to the building can also experience a significant attenuation, presumably due to the effect of the induced currents in the building. The results are supported by numerical simulations, obtained using NEC-4, in which the building is represented using a simple wire-grid model.

On Lightning Electromagnetic Field Propagation Along an Irregular Terrain

In this paper, we present a theoretical analysis of the propagation effects of lightning electromagnetic fields over a mountainous terrain. The analysis is supported by experimental observations consisting of simultaneous records of lightning currents and electric fields associated with upward negative lightning flashes to the instrumented Säntis tower in Switzerland. The propagation of lightning electromagnetic fields along the mountainous region around the Säntis tower is simulated using a full-wave approach based on the finite-difference time-domain method and using the two-dimensional topographic map along the direct path between the tower and the field measurement station located at about 15 km from the tower. We show that, considering the real irregular terrain between the Säntis tower and the field measurement station, both the waveshape and amplitude of the simulated electric fields associated with return strokes and fast initial continuous current pulses are in excellent agreement with the measured waveforms. On the other hand, the assumption of a flat ground results in a significant underestimation of the peak electric field. Finally, we discuss the sensitivity of the obtained results to the assumed values for the return stroke speed and the ground conductivity, the adopted return stroke model, as well as the presence of the building on which the sensors were located.

Evaluation of the performance characteristics of the European Lightning Detection Network EUCLID in the Alps region for upward negative flashes using direct measurements at the instrumented Säntis Tower

In this paper, we present a performance analysis of the European Cooperation for Lightning Detection (EUCLID) lightning detection network using data obtained on lightning currents measured at the Santis Tower (located in northeastern of Switzerland) from June 2010 to December 2013. In the considered period of analysis, a total number of 269 upward negative flashes were recorded at the Santis Tower. The performance of the EUCLID lightning detection network is evaluated in terms of detection efficiency, location accuracy, and peak current estimates for upward flashes. Excluding flashes containing only an initial continuous current with no superimposed pulses exceeding 2kA, the flash detection efficiency for upward flashes is estimated to be 97%. The recorded flashes contained a total of 2795 pulses (including return strokes and International Conference on Communications pulses characterized by risetimes lower than 8 mu s and peaks greater than 2kA). The overall pulse detection efficiency was found to be 73%. For pulses with peak values higher than 5kA, the pulse detection efficiency was found to be about 83%. Peak current estimates provided by the EUCLID network were found to be significantly larger than their directly measured counterparts. This overestimation might be attributed to the enhancement of the radiated electromagnetic fields associated with the presence of the tower and the mountain. The median of the absolute distance error, defined as the median distance between the Santis Tower location and the EUCLIDs stroke locations, was found to be 186m, the majority of large location errors being associated with measured current peaks lower than 10kA. The analysis revealed also that the location accuracy of the EUCLID network improved significantly in 2013 as a result of an upgrade in the location algorithms to take into account propagation effects.

Positive lightning flashes recorded on the Säntis tower from May 2010 to January 2012

Reference EPFL-ARTICLE-195165doi:10.1002/2013JD020242View record in Web of Science Record created on 2013-12-26, modified on 2017-12-10