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In electromagnetic transient analysis, one major issue is the influence of the imperfect earth on the propagation characteristics of transmission line conductors. Extensive research has been published for overhead lines, whereas the corresponding literature for underground cables is significantly less. Recently, new expressions for the calculation of the ground impedance and admittance have been proposed for the homogeneous and the stratified earth case. However, most transient simulation programs still use approximate earth representations. Scope of this study is to compare the proposed formulation with the corresponding approximations, in order to introduce a frequency limit for the use of the approximate earth models, as well as criteria that dictate the use of a stratified earth model. The resulting propagation characteristics are used in transient calculations, in order to validate the effect of the differences by the various approaches and the need to include a more accurate model in the simulation of underground cable transients.
Date of Publication: March 2013