Author: Zhang, Ge
Title: Study of the effect of observation site and propagation path on lightning-produced electromagnetic pulse (LEMP) based on rocket-triggered lightning data and FDTD code
Advisors: Chen, Mingli (BEEE)
Du, Ya Ping (BEEE)
Degree: Ph.D.
Year: 2023
Subject: Lightning
Electromagnetic pulse
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: xix, 154 pages : color illustrations
Language: English
Abstract: Lightning is a transient discharge phenomenon that can occur with long length. In nature, around one-third of lightning strikes concentrate on the earth surface, which is known as a cloud-to-ground (CG) flash. The CG flash discharge process generates a significant amount of electric current, strong electromagnetic radiation, shock waves, and other physical effects. These effects can cause severe damage to buildings and pose a significant threat to personal safety. Therefore, studying and detecting lightning is crucial for ensuring the safety of people and structures on the ground.
Nowadays, due to rapid advancements in computer technology, the finite-difference time-domain (FDTD) method has gained significant popularity as a valuable tool in the realm of lightning research. Due to the discretization of time and space by the FDTD method, this makes it very convenient and effective in studying lightning-produced electromagnetic pulse (LEMP) and its propagation over ground. Therefore, accurately representing the lightning return stroke (RS) channel in FDTD code is essential for precise calculation of the current distribution along the RS channel and the LEMPs at various distances. This thesis focuses on achieving a meticulous simulation of the lightning RS channel using the FDTD method to enhance the accuracy of lightning RS simulation and hence lightning detection and protection. Particularly, it aims to study the propagation characteristics of LEMP over ground with limited conductivities. The main research outputs are as follows:
1) A new approach for constructing a fully parameterized lightning RS channel using the FDTD method has been proposed. In this approach, five height-dependent parameters are used to control the propagation and attenuation properties of the current along the RS channel. Each parameter has a specific influence on the current distribution along the RS channel. By employing this approach, it becomes possible to exercise finer control over the current within the RS channel, leading to a more precise calculation of the LEMP.
2) Validation of the proposed model has been done with rocket-triggered lightning data obtained from Guangdong Comprehensive Observation Experiment on Lightning Discharge (GCOELD). The LEMPs calculated with the proposed model could well match with those observations obtained during the experiments, which is good indicator of the effectiveness and accuracy of the proposed model. Particularly, by fitting the calculated data with the observed data for a given RS, a relatively reasonable set of parameters of the proposed model for that RS could be determined. The channel currents calculated with this set of model parameters could reflect largely the “true” channel current distribution and propagation properties of that RS. In another words, this model could be used to retrieve the channel current attenuation and propagation properties from multi-station electric field and channel-base current measurements for a given RS, which is generally difficult to observe directly.
3) The effect of a limited ground conductivity on a LEMP as a function of the propagation distance over the ground has been investigated in detail with the proposed model. The results reveal that a limited ground conductivity leads to an extra attenuation of the amplitude and delay of the arrival time of a LEMP as it propagates over the ground, comparing with a perfectly conductive ground. In time domain, this effect manifests as an extra amplitude attenuation and arrival time delay of the peak of a LEMP waveform, while in frequency domain, it is reflected in an extra attenuation of the amplitude and phase time delay at a given frequency. For quantitative analysis, two coefficients (an amplitude attenuation coefficient and a phase/time delay coefficient) were defined in both time and frequency domains for well description of the LEMP propagation characteristics over a lossy ground. The two coefficients were finally expressed in the form of function of the ground conductivity, propagation distance and wave frequency by curving fitting of the simulated database of them.
4) Three geometry factors (the elevation of LEMP ground propagation path, the elevation of LEMP observation site, the zigzag of the lightning channel) that may affect model simulation results have been investigated. Firstly, undulating terrain is found to have a dissipative effect on the LEMP propagation. This phenomenon is quantified as a loss coefficient in the time domain, while the attenuation of amplitude and delay of the phase are summarized as a function in frequency domain. Secondly, the height of the building where the detection antenna is located is found to have a strong amplification effect on the received signal. An enhancement coefficient is defined to describe this phenomenon, which is found to be proportional to the height of the antenna location. Besides, an antenna located around the corner at a building roof has a stronger amplification of the received signal compared to the one located at the center of the building roof. Thirdly, it is found that a zigzag RS channel in FDTD simulation has a significant impact on the simulation result. Bends of a RS channel not only increase the channel effective length but also change the relative propagation distance of the lightning-generated electromagnetic waves to the observation point compared to a vertical channel case. This means that a zigzag channel may result in a large amplification or attenuation of the simulated LEMP.
Rights: All rights reserved
Access: open access

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