2-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 statio n grounding systems. Calculations based on such models usually give co rrect values for the gound resistance of grounding electrodes and for the step and touch potentials on the surface of the ground. However, t he two-layer model of the actual soil can give absurd results if incor rect values of the layer resistivities and the first layer thickness a re used. Conventional methods of fitting the apparent resistivity curv e 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 ce rtain combinations of different layer resistivities exist. In this pap er, an analysis of critical cases was carried out, and a novel calcula tion method for the parameters of the two-layer soil model is presente d. 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 me thod were compared with electromagnetic field calculations carried out with a computer program based on the finite-element method. The propo sed 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, th e risk of serious calculation errors that may appear by application of the apparent resistivity fitting method can be avoided.