Dependência da frequência dos parâmetros do solo: efeito no comportamento impulsivo de aterramentos elétricos

Abstract

This PhD dissertation is dedicated to the study of the frequency dependence of soil electrical parameters and to the analysis of the impact of such phenomenon on the grounding response to transients, namely those associated with lightning. A new methodology to determine the frequency dependence of soil parameters based on measurements in field conditions is proposed. The methodology is able to determine the frequency dependence of soil parameters in 100 Hz to 4 MHz range, which corresponds to the typical range of lightning current frequency components. The results provided by the developed methodology are validated and the relevance of the frequency dependence of soil parameters on the impulse grounding behavior is experimentally proved. The methodology is systematically applied to more than 60 soils in natural conditions in order to generally characterize the frequency dependence of soil parameters. It is shown that the soil resistivity decreases with increasing frequency and that such decrease is more significant for soils of higher low-frequency resistivity. It is shown that the permittivity decreases with increasing frequency and that it presents, in most part of spectrum, values that are much higher than those usually assumed in literature. Based on the experimental results, formulas to estimate the frequency dependence of soil parameters are developed. Using these formulas, the impact of the frequency dependence of soil on the transient response of grounding is assessed by means of computational simulations. It is shown that the frequency dependence of soil is responsible for decreasing the grounding potential rise of electrodes and, thus, their impulse impedance and their impulse coefficient. This effect is more pronounced with increasing low-frequency soil resistivity and for typical currents of subsequent strokes. Furthermore, an original physical model to characterize the frequency dependence of soil parameters, based on Kramers-Kronig relationships and Maxwell equations, is proposed.