Earth Return Admittance Effect on Underground Cable System Modeling

Typically, in cable system modeling for power system transients studies, the ground return effect is taken into account in the per unit length impedance, its effect on the admittance being neglected. In this paper, the per unit length impedance and admittance matrices for a cable system are established, based on the approach previously presented by the authors for the case of a single insulated cable in a dispersive medium. The presented formulation is derived from a full-wave approach, assuming quasi-TEM propagation which was proved suitable for the frequency range of interest. A comparison of the propagation modes indicates that the inclusion of the ground return admittance affects significantly the ground and intersheath modes in the high frequency range, i.e., above a few kilohertz. Test cases where the different modes are excited are used to analyze the impact of including the ground return admittance in computing the transient response of underground cable systems. The results indicate that this impact increases for high resistivity soils and shorter cables.