Effect of approximate impedance formulae on the accuracy of transmission line modelling

It is well known that the accuracy of frequency-dependent line models in time-domain calculations relies on the equations that define the line propagation matrix H and characteristic admittance Yc. Therefore both modal-domain and phase-domain transmission line models are affected by the assumptions in the evaluation of the series impedance Z, namely, the formulation of the internal conductor and the ground return impedance. The complexities of the exact representation of these parameters have led investigators to propose approximate formulae, and several of these are available in the technical literature. This work examines the effects of these formulae on the calculation of matrices H and Yc on the time-domain responses of the line. Several possibilities have been tested. The results show that although the approximate methods can be used for overhead lines, caution is needed for underground cable systems.