Accuracy limits in the computed transients on overhead lines due to inaccurate ground return modeling

Electromagnetic transients on overhead transmission lines are strongly affected by the resistivity ρ of the ground return which, however, is often poorly known. Therefore, the calculation of transients with very high accuracy for a given ρ may not make good engineering sense if it entails loss of computational efficiency. The effect of ρ on the transients comes primarily via the ground return impedance. In order to judge what accuracy is reasonable, this paper derives the sensitivities of the ground return impedances for line and ground modes with respect to the resistivity ρ. The results are simple and may assist decidions on the accuracy of computations in EMTP-type simulation programs.