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|Title:||Study of propagation of lightning-produced electromagnetic impulse (LEMP) on earth surface based on practical lightning location network (LLN)||Authors:||Ding, Xueyun||Keywords:||Lightning protection.
Hong Kong Polytechnic University -- Dissertations
|Issue Date:||2013||Publisher:||The Hong Kong Polytechnic University||Abstract:||Lightning, especially the cloud-to-ground (CG) lightning, is making massive damages to humans, environment and electric and electronic devices every year. A kind of Lightning Location Network (LLN), based on the combination of MDF (magnetic-direction-finder) and TOA (time-of-arrival) techniques, has been world-widely used to provide the location and current amplitude of CG lightning strokes occurred so that to minimize the lightning-caused disasters. However, by assuming that the earth is a smooth and perfect conductive sphere, most of the existing LLNs are designed and operate without considering the influence of the path roughness on the propagation of lightning-produced electromagnetic impulses (LEMP). This study aims to figure out the effect of earth surface roughness on the propagation of LEMP based on abundant experimental data from the LLN. Theoretically, the ground condition may affect a LEMP's amplitude and propagation speed. To describe the path effect on LEMP, two new parameters are defined: the "path effect coefficient" that is associated to the LEMP amplitude and the "arrival time delay" that is associated with the LEMP propagation speed. A new approach based on statistical analysis of the LEMP amplitudes and its arrival times recorded by individual sensors in a LLN is proposed to estimate the spatial profiles of the two newly defined parameters for each sensor in the LLN. This approach is applied to a Chinese regional LLN consisted of 25 TOA/MDF sensors and the results are analyzed in three formats: i) the spatial distribution of the "path effect coefficient" and "arrival time delay" versus the 3-D terrain topography for each sensor, ii) their averages versus the distance to the sensor, and iii) their distributions along a specific earth path for a specific sensor in the LLN. On one hand, the results show that the average "path effect coefficient" is about 1 near a sensor and has a general trend of decrease superposed with significant fluctuations as the distance increases. On the other hand, the average "arrival time delay" shows an almost linear increase versus the distance, which increases from about 0.5~1 μs nearby the sensor up to about 10 μs when 600 km away from the sensor. There are also small fluctuations superposed on the straight increasing line of the "arrival time delay" and these fluctuations are more obvious when the corresponding terrain topography changes greatly.
Theoretical explanations for these findings have been explored as following. Firstly, the earth is not smooth. The reflected electromagnetic wave by earth protuberance will splice the original wave, resulting in changes in both the LEMP amplitude and its arrival time reading. Secondly, there is a vegetation layer on the earth. For example, a LEMP propagating over a forest may get extra polarization on tree trunks, which will affect the equivalent dielectric constant, and further affect the LEMP group velocity and hence its arrival time. Different vegetation will get different conductivity and equivalent dielectric constant. Thirdly, the distances on phase angles of the main frequencies dominating the LEMP peak will also affect the LEMP peak received by a sensor. These findings are helpful in at least two aspects: the corrections of the peak current and location of lightning strokes detected by a LLN in practical aspect, and the understanding of the physical rules of LEMP propagating over rough earth surfaces in theoretical aspect.
|Description:||xii, 123 p. : ill. (some col.) ; 30 cm.
PolyU Library Call No.: [THS] LG51 .H577M BSE 2013 Ding
|URI:||http://hdl.handle.net/10397/6377||Rights:||All rights reserved.|
|Appears in Collections:||Thesis|
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