Numerical Electromagnetic Analysis of Lightning Surge Response on Vertical Conductor

Abstract

In analyzing the lightning performance of overhead power transmission lines and substations, the lightning surge characteristics of transmission line components as well as the statistical data of lightning such as the ground flash density and the stroke peak current distributions are essential. Among the transmission line components, tower surge characteristics including the tower footing impedance characteristics in the linear region are probably the most fundamental factors since they contribute directly to the insulator voltages during a lightning hit. Particularly for such tall structure as EHV or UHV doublecircuit towers, the characteristics become more dominant owing to the longer round-trip time of a travelling wave in the tower. An agreement on the interpretation of this phenomenon, however, has not been reached yet. There have been three considerations to evaluate the transient characteristics of a tower as follows: (i) theoretical studies, (ii) simulation of reduced scale models and (iii) simulation of full-sized tower. Measurement on full-sized towers is straightforward in evaluating the actual characteristics of a tower struck by lightning, however it is difficult to cam' out this kind of experiments in the ideal arrangement where a current lead wire is stretched vertically above the tower top to simulate a lightning channel, owing to its scale. Measurement on reduced scale models is more economical than full-sized towers, and is flexible in setting up various experimental arrangements. It is however not easy to maintain the accuracy. Theoretical studies on simplified geometry may be useful in understanding the phenomenon, however, they are invalid for the accurate evaluation of the dynamic electromagnetic behavior of a three dimensional (3-D) system struck by lightning. The simulation analysis of surge response are carried out in the several arrangements of the current lead wire: (i) vertical and at the top of vertical conductor. (ii) vertical and a little far from the top of vertical conductor. and (iii) horizontal and far from the top of vertical conductor. In all the cases, the voltage measuring wire is placed at the perpendicular to the current lead wire. Each of the arrangement of the current lead wire affects the measured surge impedance of the vertical conductor and that has been explained in this research in detail. If a travelling wave propagates along the vertical conductor at the velocity of light, the reflected wave from the ground should return to the .4 top of the vertical conductor just after the round-trip time of the travelling wave in the vertical conductor. For the accurate analysis of the dynamic electromagnetic field around a complex tower system struck by lightning, electromagnetic modeling codes are appropriate. Of many available codes, the Numerical Electromagnetic Code (NEC-2) based on the Method of Moments is chosen for the present work, since it has been widely and successfully used in analyzing thin-wire antennas and a tower system can be regarded as a thin-wire antenna. In the present thesis, firstly, the applicability of NEC-2 to the electromagnetic field analysis of vertical conductor surge response is verified by comparison with theoretical results on simple structure. Secondly, four parallel conductors model are taken into account for the analysis by NEC-2 to evaluate the surge characteristics. Thirdly, the surge characteristics of actual tower struck by lightning are studied with the help of NEC-2. Finally, it can be concluded that this research presents some results concerning the simulation of electromagnetic transient in transmission lines caused by direct and indirect strikes of lightning, which will help to design the structure of tower and to design lightning arrester to protect various electric appliances that is used in our everyday life.