Tower and Path-Dependent Voltage Effects on the Measurement of Grounding Impedance for Lightning Studies

We present an analysis aimed at evaluating the effect of the presence of the tower on the measurement of the grounding impedance. The Numerical Electromagnetics Code NEC-4 is used for the full-wave modeling of the structure including the tower and its footing grounding system, as well as the experimental setup used for the measurement of the grounding impedance. First, the effect of possible path-dependence in the evaluation of the integral of the electric field is investigated. It is shown that, up to a frequency of about 100 kHz or so, the voltage, and consequently the evaluated impedance are independent of the path of integration. At higher frequencies, differences appear, which might be significant especially for poor conducting ground. The effect of the distance to the reference remote earth on the evaluated voltage is also analyzed and it is shown that a distance of 100 m to the remote earth is appropriate to evaluate the grounding impedance potential rise. The simulation results show that at low frequencies (up to about 200 kHz or so for the considered 140-m tall tower), the effect of the presence of the tower can be disregarded. At higher frequencies, the evaluated impedance is affected by the presence of the tower. The poorer the ground conductivity, the more significant the effect of the presence of the tower on the evaluated impedance. Two injected current waveforms, representative of typical first and subsequent return strokes, are considered for the time-domain analysis. The effect of the presence of the tower can be significant for subsequent return stroke excitation, which has a wider frequency spectrum compared to first return strokes. It is shown that in the early-time region, part of the injected current flows into the tower. For a ground conductivity of 0.001 S/m, the tower peak current increases to about 10% of the exciting current, for the case of a subsequent strokes. It is also shown that the presence of the tower would cause a decrease in the peak potential rise leading therefore to an underestimation of the so-called impulsive grounding impedance.