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

• Published in: IEEE transactions on electromagnetic compatibility Vol. 47; no. 1; pp. 146 - 159
• Main Authors: Bermudez, J.L., Rachidi, F., Rubinstein, M., Janischewskyj, W., Shostak, V.O., Pavanello, D., Jen Shih Chang, Hussein, A.M., Nucci, C.A., Paolone, M.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01.02.2005; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Applied sciences; Electric fields; Electromagnetic compatibility; Electromagnetic fields; Electromagnetic reflection; Electromagnetism; Elevated; elevated strike objects; Exact sciences and technology; Information, signal and communications theory; Lightning; lightning location systems; lightning return stroke modeling; Lightning strikes; Magnetic fields; Mathematical models; Return strokes (lightning); Strikes; Studies; Telecommunications and information theory; Towers; Transmission line theory; elevated strike objects; Far field; Electromagnetic compatibility; Electromagnetic fields; Modeling; Transmission line; Lightning; Measuring system; Current distribution; lightning location systems; Electromagnetic field; lightning return stroke modeling; Return stroke
• ISSN: 0018-9375; 1558-187X
• DOI: 10.1109/TEMC.2004.842102

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.