Homogenous Earth Approximation of Two-Layer Earth Structures: An Equivalent Resistivity Approach

The homogenous earth representation of two-layer earth structures for earth return impedance calculations is investigated in this paper. This representation is based on equivalent resistivity, which takes properly into account the electromagnetic and the geometrical properties of the two earth layers. The equivalent resistivity can be used in the relatively simpler formulas for the earth return calculations for the case of homogenous earth. The new expression is implemented for six actual cases of two-layer earth structures involving combinations of overhead and underground conductors. Results show that the equivalent resistivity approach can lead to significant simplifications in most cases of switching transient simulations in the presence of two-layer earth structures