Two-layer soil model for power station grounding system calculation considering multilayer soil stratification

Two-layer soil models are commonly used for the design of power station grounding systems. Calculations based on such models usually give correct values for the gound resistance of grounding electrodes and for the step and touch potentials on the surface of the ground. However, the two-layer model of the actual soil can give absurd results if incorrect values of the layer resistivities and the first layer thickness are used. Conventional methods of fitting the apparent resistivity curve in order to obtain those parameters do not give good results in all cases. This may happen when the soil has a multilayer structure and certain combinations of different layer resistivities exist. In this paper, an analysis of critical cases was carried out, and a novel calculation method for the parameters of the two-layer soil model is presented. The proposed method is valid for an arbitrary number of soil layers with arbitrary values of resistivity, and it is applicable to simple as well as complex grounding systems. The numerical results of this method were compared with electromagnetic field calculations carried out with a computer program based on the finite-element method. The proposed model permits the use of computer programs based on the method of images and a two-layer soil model for the design of station grounding systems buried in horizontal multilayer soil stratifications. Thus, the risk of serious calculation errors that may appear by application of the apparent resistivity fitting method can be avoided.