|Title:||Lightning surges on vertical structures|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Building Services Engineering|
|Pages:||xi, 239 leaves : ill. ; 30 cm.|
|Abstract:||Lightning is a serious weather hazard, which not only causes unpredictable power interruptions and serious damage to the structures on the ground, but also imposes a threat to livestock and human beings. To provide effective protection scheme, it is necessary to carry out the analysis on lightning surge. In the past decades, the characteristics of the surge on horizontal structures such as overhead transmission lines have been well addressed. The surge propagation on the horizontal wires is in TEM mode, and can be illustrated by applying the traditional transmission line theory. However, the surge propagation on vertical structures is not in TEM mode. The characteristics of the surge on vertical structures have not been presented well in the literature. To investigate lightning surges on vertical structures, the full-wave numerical method based on the Partial Element Equivalent Circuit (PEEC) was developed. By applying this method to the modeling of the vertical structures, the potential and current on the vertical structures are obtained, and the characteristics of the lightning surges are discussed. The proposed numerical PEEC method is verified by NEC2. To provide an easier way for the lightning surge analysis on vertical conductors, the Traveling Wave Theory for Vertical Conductors is proposed. In this theory, the theoretical formulas for calculating surge impedances of vertical conductors are derived. The current attenuation along the conductors is discussed and then the effect of the current attenuation on the calculated surge impedance is illustrated. An iterative method for calculating surge impedance and current attenuation is then proposed. Moreover, the characteristics of the surge behavior at discontinuities of vertical conductors are investigated. The analysis shows that the transmission equations of the traditional transmission line theory can be applied, as long as the surge impedances are redefined according to the Traveling Wave Theory for Vertical Conductors. The proposed Traveling Wave Theory for Vertical Conductors is justified by the numerical simulation based on PEEC method.|
After that, the induced surge in buildings is investigated. Firstly, on horizontal circuits above the perfect ground, the equations of the induced surge on distribution wires are derived. The factors affecting the induced surge are discussed. The effect of the load type on the induced surge is illustrated. The multi-reflection equations of the induced surge are derived. The total induced surges on the distribution wires are then investigated numerically, and the corresponding protection schemes are proposed. The effect of the lossy ground on the induced surge is introduced. Secondly, the surges on the rising bus are investigated numerically. The factors affecting the induced surge on vertical conductors are discussed. The total induced voltages on the rising bus and the connected distribution wires are investigated. The protection scheme is introduced. Apart from the analysis above, the current sharing among tower mounted cables is also investigated. This investigation is carried out by a hybrid-numerical method based on PEEC and EMTP. The tower and the cables mounted on it are modeled in PEEC. The numerical model is then solved by EMTP. By investigating the current from the simulation results, it is shown that the arrangement of the cables can affect the current sharing. The unbalanced currents in the cables should be paid attention in the design of protection schemes.
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