Influence of Earth Permittivity on Overhead Transmission Line Earth-Return Impedances

Earth displacement currents have a significant effect in the analysis of high frequency phenomena. In this paper, the influence of the earth permittivity on the overhead power transmission line impedances is investigated. Certain criteria are proposed for the classification of cases where displacement currents must be taken into account. A systematic comparison of the existing approaches for the homogenous earth models is done, while the accuracy of the results is justified using a proper finite- element method formulation. A typical 3phase, single circuit line configuration is examined, for a combination of earth permittivities and resistivities over a wide frequency range. The differences in the transmission line impedances and the propagation characteristics, due to the impact of displacement currents are presented. Finally a simple switching transient simulation is examined to evaluate the influence of different earth permittivities on the transient voltages and currents.