Validation Limits of Quasi-TEM Approximation for Buried Bare and Insulated Cables

For the analysis of power system transients in overhead transmission lines and underground cables, a common assumption is related to the quasi-TEM (transverse electromagnetic) approximation. In this paper, we investigate the applicability of this approximation throughout a wide frequency band when both conduction and displacement currents in ground are considered, typically only the former is considered. A full-wave model is established to assess the behavior of the propagation constant for bare and insulated underground cables in a wide frequency band and verify the adequacy of the quasi-TEM approximation. The results demonstrate that it is possible to retain quasi-TEM approximations in the high-frequency range as well. One has only to consider different assumptions for the propagation constant. For the evaluation the adequacy of the quasi-TEM approximation, a simple, single-phase buried cable is considered. The effect of the cable coating is also taken into account and the results indicate that the quasi-TEM approximation is rather distinct for bare or insulated cables. The accuracy of the approximation is evaluated from comparison of step and impulse voltage tests considering different soil models.