|Title:||Electromagnetic transient analysis for lightning strikes on a grounded structure|
|Advisors:||Du, Yaping (BSE)|
Building -- Protection
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Building Services Engineering|
|Pages:||126 pages : illustrations|
|Abstract:||Lightning is a powerful natural phenomenon. It could cause fatal casualties, severe damage to the facilities, and mal-function of electrical or electronic systems. Lightning protection, therefore, is imperative for the facilities and systems on the ground. To design efficient and effective protection measures, transient analysis, and evaluation of lightning strikes on a ground structure is considered essential. This thesis firstly introduces an extended time-domain traveling-wave (TDTW) theory which can be applied to the lightning transient analysis. It is based on the wave equation for scalar potential and vector potential and the retarded Green's equation. The traditional traveling wave theory assumes the constant impedance and reflection coefficient of the transverse electromagnetic (TEM) traveling wave. This is not in coincidence with the non-TEM propagating behavior of the lightning current surge along the lightning path. The proposed TDTW theory introduces time-and-spatial-variant impedance and time-variant reflection coefficient in a simple format. Furthermore, the traveling waves can be divided into primary waves and secondary waves in the presence of discontinuities or lumped circuit elements. The overall solution is simple and analytical under the ramp waveform source. To adapt it to arbitrary waveform excitation, a convolution technique is adopted. The TDTW theory would benefit in constructing a more reasonable lightning engineering model to obtain the corresponding lightning-induced electromagnetic field. Meanwhile, the TDTW theory can be used in determining the transient of a traveling wave antenna with lumped loading. This thesis also proposes a novel lightning electromagnetic model, which is the hybrid model of distributed circuit (DC) model and the partial element equivalent circuit (PEEC) method. It is well known that most of the physical characteristics of the lightning channel evolution can be reproduced by the DC model in the simplest way, while the electrical circuit behavior of complicated wire ground systems can be dealt with by the PEEC method. Thus, this hybrid electromagnetic and circuit model can be used to model the physical behavior of the lightning channel and the circuit characteristics of a complicated ground structure together. Most recently, the physical dynamic characteristics of the lightning corona effect are well established. Thus, in this thesis, the exited DC model is extended to include the non-linear and non-uniform behavior of the dynamic corona effect. Meanwhile, because the initial charge also plays an important role in the physical evolution, the modified charge simulation method (CSM), which is modified to involve the dynamic corona effect, is proposed to obtain the initial charge distribution beneath the thundercloud. The modified CSM is solved with Newton's iterative method. The hybrid electromagnetic and circuit model is solved with Piecewise Linearization (PL) Method and march on-in-time technique. The simulation results are well matched with the experimental measurement data. This model exhibits the influence of various ground structures and dynamic corona effect on the lightning current surge and corresponding LEMP. In general, this thesis proposes an extended traveling wave theory and a hybrid electromagnetic and circuit model. Both can be used in lightning strikes on a ground structure issue. The former is of simple and closed-form expression and would benefit from giving a more reasonable engineering model. The latter is based on the electromagnetic numerical method and aims at proposing a novel electromagnetic model to include both the physical behavior and circuit characteristics of a lightning strike. The choice of an appropriate model highly depends on the research objective and specific engineering application.|
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