On return stroke currents and remote electromagnetic fields associated with lightning strikes to tall structures: 2. Experiment and model validation

Abstract

[1] In this paper, simultaneous GPS time-stamped measurements of the electric and magnetic fields at three distances and of the return stroke current associated with lightning strikes to the Toronto CN Tower (553 m) during the summer of 2005 are presented. The lightning return stroke current was measured using a Rogowski coil installed at a height of 474 m above ground level (AGL). The vertical component of the electric field and the azimuthal component of the magnetic field were measured simultaneously at distances of 2.0 km, 16.8 km, and 50.9 km from the CN Tower. The propagation path from the CN Tower to the first two stations (2.0 and 16.8 km) was along the soil and through the Toronto city, whereas for the third location (50.9 km) the propagation path was nearly entirely across Lake Ontario. The waveforms of the electric and magnetic fields at 16.8 km and 50.9 km exhibit a first zero crossing about 5 ms after the onset of the return stroke. This early zero crossing is part of a narrow undershoot. For fields at 50.9 km the expected zero crossing at about 40 ms is also observed. Metallic beams and other conducting parts in buildings on which electric and magnetic field sensors were located cause an enhancement effect on the measured fields. Although an enhancement can be identified both on the electric and the magnetic fields, the degree of enhancement is actually more significant for the electric field than for the magnetic field. It is shown that the value of the wave impedance (E-field peak to H-field peak ratio) could give an estimate of the enhancement effect of the building on the electric field. Propagation effects (decrease of field amplitude and increase of its risetime) can also be observed in experimental records. It is shown that the fields at 50.9 km are less affected by such attenuation, compared to those at 16.8 km, presumably because the path of propagation is mostly across Lake Ontario. Measured waveforms are compared with theoretical predictions obtained using the five engineering return stroke models extended to include the presence of the strike object, namely, transmission line (TL), modified transmission line (MTLL and MTLE), Bruce-Golde (BG), and traveling current source (TCS) models. A reasonable agreement is found with all five engineering models for the magnetic field waveforms at the three considered distances, although the peak values of the computed fields are systematically about 25% lower than measured values. None of the models was able to reproduce the early zero crossing and the narrow undershoot. As far as the electric field is concerned, larger differences have been observed between simulations and measurements. This may be due to the fact that the enhancement effect of the building on the electric field is stronger than that on the magnetic field. The expression relating current and field peaks associated with strikes to tall structures is also tested JOURNAL

Effect of Nearby Buildings on Electromagnetic Fields from Lightning

“…As discussed in [7], the obtained results suggest an enhancement of the vertical electric field measured on the roof of the building, in line with the conclusions of [1][2][3][4][5][6]. The enhancement referred to in the mentioned studies is defined as the ratio of the fields on top of a building to the corresponding fields in the absence of the building.…”

“…Metallic beams and other conducting parts in those structures may cause enhancement or attenuation effects on the measured fields [1][2][3][4][5][6]. Rubinstein et al [1] used simultaneous measurements of lightning electric fields at the top of a building and at ground level to estimate an enhancement factor for the electric field of about 1.5 for their 17-floor building.…”

“…Their results imply that the enhancement factor for the electric field is about 2.3 for a 10-m high building and increases with the height of the structure. Bermudez et al [4] and Pavanello et al [5] compared electromagnetic fields associated with lightning strikes to the Toronto CN Tower measured on the roofs of buildings at different distances from the tower with theoretical estimations. Their results suggest that both the electric and the magnetic fields may have been enhanced by the presence of the buildings, although the degree of enhancement was actually more significant for the electric field than for the magnetic field.…”

Lightning strikes to tall objects: A study of wave interactions at the return‐stroke front using a nonlinear transmission line model

“…some engineering return-stroke models extended to account for the interaction of lightning with towers are formulated so that reflected current pulses transmitted from the tower to the channel travel faster than the return-stroke front without experiencing attenuation or distortion. As a consequence, these pulses eventually catch up with the return-stroke front, resulting either in a current discontinuity (if the current pulses transmitted from the tower to the channel are assumed to vanish instantaneously when arriving at the upward propagating front) [Pavanello et al, 2004[Pavanello et al, , 2007a[Pavanello et al, , 2007b or in current reflections (if an impedance discontinuity is assumed at the upward moving front) [Janischewskyj et al, 1998;Shostak et al, 2000;Boev and Janischewskyj, 2011;Mosaddeghi et al, 2011;Rahimian and Hussein, 2012], depending on model considerations. In the latter case, both channel and tower currents are modified by reflected current pulses that propagate downward from the upward moving front as shown by the dashed lines in the diagram of Figure 1.…”

“…In general, engineering return-stroke models that include the possibility of current reflections at the upward moving return-stroke front as well as the presence of an upward connecting leader [e.g., Shostak et al, 2000;Mosaddeghi et al, 2011] are believed to reproduce the fine structure of current waveforms and remote electromagnetic fields associated with lightning strikes to tall towers better than models that neglect such factors [e.g., Pavanello et al, 2007aPavanello et al, , 2007b, even though a constant reflection coefficient at the upward moving channel front is usually assumed for that purpose (a time-variant current reflection coefficient should be expected at the end of an extending transmission line as discussed by Shoory et al [2011Shoory et al [ , 2012) and current attenuation and distortion due to channel losses are ultimately disregarded. More recently, an electromagnetic return-stroke model considering corona and nonlinear losses was used to investigate the interaction of lightning with an elevated strike object [Raysaha et al, 2012].…”