Development of a 3-D leader propagation model and its application to structural lightning protection analysis

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Development of a 3-D leader propagation model and its application to structural lightning protection analysis

 

Author: Xu, Yazhong
Title: Development of a 3-D leader propagation model and its application to structural lightning protection analysis
Degree: Ph.D.
Year: 2013
Subject: Lightning protection.
Lightning.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xviii, 170 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2653096
URI: http://theses.lib.polyu.edu.hk/handle/200/7295
Abstract: Lightning is one of the most mysterious yet deleterious and disastrous natural phenomena on earth. It is a major threat to the natural resources that we rely on and material properties which human create. In addition, there are quite a lot of human and animal casualties caused by lightning strikes around us each year. Thus, there is a need tor lightning physics and protection research in order to enhance the understanding of lightning striking mechanism, so as to design lightning protection for both living beings and things such as buildings and utility poles, etc. Apart from field observation of natural and triggered lightning and laboratory long spark experimentation, many current theoretical lightning studies have been dealt with both laboratory spark and field discharge by means of numerical modeling. However, few of them are able to connect theoretical modeling to practical application properly. Therefore, it is highly desirable to develop a numerical lightning model that can describe physical processes well while at the same time serve as a virtual tool for engineering lightning protection evaluation and design. The thesis presents, first of all, a 3 dimensional (3-D) self-organized leader propagation model (LPM) for downward stepped leader in negative cloud-to-ground (CG) lightning. The characteristic features of stepped leaders are generalized. So that leader behavior such as bending, step advancement, charge distribution and corona sheath variation are described by a series of equations and criteria, leading to a stochastic and step-wise growth of 3-D leader channels. Lightning parameters, such as leader charge distribution, corona sheath radius, core tip potential, ground electric field strength, step time interval, leader speed, leader current and etc. are calculated accordingly. Parameters predicted by the proposed LPM are found to be in close agreement with those of existing models. Besides, striking point probability concept is introduced based on the present model and found to be potentially useful.
Secondly, for the convenience of the engineering application of lighting protection evaluation and design, the LPM needs to be simple and intuitive. The model has been set to operate under various situations at different leader initiation height and electric potential. Based on the statistical pattern of the generated results, the relationship between the height and potential of leader initiation in the cloud and the charge distribution along the leader channel has been derived, which is part of the so-called engineering approximation. Then the lightning striking distance to flat ground is defined and calculated with the help of the engineering approximation. Subsequently, The striking distance has been connected to the return stroke peak current based on appropriate physical understanding. It is concluded that the striking distance to flat ground is more associated with charge density of the leader tip rather than total charge along the leader channel. This is because total charge along the leader channel varies with both height and potential of the leader initiation conditions, while the leader tip charge density is only closely correlated with leader initiation potential, At last, with the help of aforementioned leader propagation model and its engineering approximation, engineering structural lightning protection analysis becomes feasible. In the present thesis, lightning rods and buildings have been dealt with the present method. On the one hand, various lightning rods with different heights are analyzed under various conditions provided by our method. Engineering lightning protection parameters are defined and calculated, i.e. the protection range and the protection angle. They are determined by the striking distance range and the maximum attractive boundary of the lightning rods under lightning strikes of various strength, i.e, return stroke peak current. The relationships between these parameters and return stroke peak current are then presented. Striking distances to the lightning rods calculated by the present method are shorter than IEC 62305 suggests, indicating the present method is more conservative. One the other hand, a few buildings with varying dimension have also been conducted with the present method. Similarly, engineering lightning protection parameters are defined for example, the attracting volume of the structure, the shielding angle and the collection range. Therefore, being physically sound and mathematically concise, it is potentially useful to apply the present method for engineering structural lightning protection practice.

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