Extension of a modal-domain transmission line model to include frequency-dependent ground parameters

• Transmission line model is extended to include frequency-dependent ground parameters.
• Transients in power distribution lines are calculated in time domain.
• Proposed model is adequate for high-frequency transients and poorly-conducting ground.
• Constant ground parameters can be sufficiently accurate if the ground permittivity is suitably selected.
• Transmission line branches can reduce the importance of frequency-dependent ground model.

This paper investigates the effect of frequency-dependent ground parameters on the simulation of electromagnetic transients on overhead power distribution lines using a modal-domain based transmission line model available in popular electromagnetic transient programs. The line parameters are calculated considering a soil model that is based on field measurements of ground conductivity and permittivity in a wide frequency range. An assessment is made of the errors associated to assuming constant or frequency-dependent ground parameters in the calculation of the ground-return impedance and the ground admittance correction in the case of a poorly conducting ground. The propagation function and the characteristic impedance of the transmission lines are fitted in the frequency domain as the sum of rational functions. The associated poles and residues are written in a .pch file that is interpreted by the Alternative Transients Program (ATP) as a frequency-dependent line model. Time domain simulations are performed considering different types of transients on typical overhead power distribution lines. The results indicate that the consideration of frequency-dependent ground parameters can be relevant in the simulation of high-frequency transients on transmissions lines if the ground is a poor conductor, although the presence of line branches is likely to minimize the observed effects. It is also shown that constant values of ground conductivity and permittivity are able to lead to results comparable to those obtained with a dispersive ground model provided a suitable value is selected for the ground relative permittivity.