Theoretical Simulation and Experimental Investigations of the Surge Response of a Tower Model of Vertical Conductor

The steel tower surge impedance is one of the basic parameters for the anti-lightning design. Therefore, since Jordan, a lot of experiments and theories are proposed, however, there are no established theories. This is the stumper which is known as ``the vertical conductor problem'' in the present-day electric engineering. Therefore, it is given up to make the problem clear, and the present-day situation is about to adopt the numerical analysis which can explain the phenomenon comparatively well. As for surge impedance on a tower model of a vertical conductor, we have a theoretical formula by Lundholm. The theory of Lundholm looks like to be a perfect theory, however, this formula does not coincide with the experimental results. Thus, there is the strange situation that the Jordan's formula of the wrong theory agrees with the experiments more correctly. Hara et al. derived experimental formula. Moreover, one of the authors proposed a theoretical formula of surge impedance; considering the existence of ground surface, and without ground surface. The former formula is very similar to the experimental formula of Hara et al.. In this research, these theoretical formulas of surge impedance are examined by the simulation analysis of vertical conductor with the help of Numerical Electromagnetic Code (NEC-2) and the experimental results of that. In the measurement at an actual tower, however, it is difficult to stretch a current lead wire vertically from the tower top, where the current lead wire acts as a vertical lightning channel. Measurements on reduced-scale models are more economical than those on full-sized towers, and are flexible in setting up various experimental arrangements. It is, however, not easy to maintain the accuracy of the measurement, since the geometrical size of the measuring devices is large relative to the measured system. The simulation and experimental analysis of surge response are carried out in the several arrangements of the current lead wire and the current source: (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 these will be 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 top of the vertical conductor just after the round-trip time of the travelling wave in the vertical conductor. We ascertained these phenomenon in the both measured and computed results. The agreement between the measured and computed results is also quite well with minor difference. The experimental set up of the model to be analyzed in this paper is verified with the simulation result of the equivalent circuit model by the EMTP.