Analysis of ground return impedance calculation methods for modeling of underground cables for lightning studies

Analysis of lightning transients in cable systems necessitates accurate calculation of ground return impedances (GRIs) in high-frequency regions. EMT-type programs used for transient analyses incorporate various methods for calculation of GRI. Most of these methods include assumptions and approximations, the validity of which needs to be questioned at high frequencies. In particular, the common approximation of the neglecting effect of displacement currents should be reviewed. The purpose of this paper is to evaluate commonly used GRI calculation methods in terms of their accuracy and validity in high-frequency regions. In this study, GRI calculation methods are analyzed and compared, the effect of displacement currents is evaluated, and validity ranges of the basic calculation formula (Pollaczek formula) are discussed. It is shown that for high-resistivity and high-permittivity soil cases, the effect of displacement currents must be taken into account in high-frequency regions. Most default calculation routines in EMT-type programs neglect this effect. Therefore, it is very important to review and understand the calculation methods used in EMT-type programs before employing them in high-frequency transient analyses. Moreover, it is shown that in most of the practical cases the basic calculation formula (Pollaczek formula) is valid for the frequency range of interest for lightning studies.

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