Multilayer Earth Structure Approximation by a Homogeneous Conductivity Soil for Ground Return Impedance Calculations

This paper proposes a technique to approximate a multi-layer earth structure by a homogeneous conductivity soil in ground return impedance calculations. An equivalent real-valued conductivity parameter is obtained, which can be used with reasonable accuracy in a simpler expression or in commonly available EMTP software, such as the Line Constants routine of the Alternative Transients Program (ATP). Several actual soil models are evaluated at frequencies ranging from 1 Hz to 2 MHz. Results show that the proposed method is accurate for modeling most power system problems, from steady-state conditions to transients commonly verified in electrical systems. The proposed expression is easy to use and introduces a considerable performance gain in terms of floating-point operations, compared to the analytical formulation of the general multi-layer soil structure.