External Impedance and Admittance of Buried Horizontal Wires for Transient Studies Using Transmission Line Analysis

The paper investigates the applicability of some closed form expressions for the ground impedance and ground admittance of buried horizontal wires (bare and insulated) for lightning or switching transient analyses based on transmission line (TL) theory. In view of the frequency contents that typically characterize such transients, the behavior of the ground impedance and admittance is studied for a wide frequency range up to 10 MHz. Low frequency approximation of the ground impedance is not always appropriate for transient analysis. Sensitivity analyses show that, unlike overhead wires, the ground impedance for buried wires is little sensitive to the ground conductivity. On the other hand, the ground admittance varies strongly with the ground conductivity. The paper also discusses the results of transient analysis of buried cables performed by means of electromagnetic transient programs (EMTP) that neglect the ground admittance. The limits of such an approximation are discussed in order to evaluate the applicability of EMTP-like programs to the transient analysis of buried conductors. Transient pulse propagation in time domain based on finite difference time domain (FDTD) method of solution of TL equations is also discussed for a future inclusion of non-linear phenomena, like soil ionization and arcing/breakdown mechanisms, in the soil. The analysis presented could be useful in estimating surge propagation characteristics of buried wires for appropriate insulation coordination and transient protection.