A BEM formulation for computational design of grounding systems instratified soils

Abstract. Substation grounding design involves computing the equivalent resistance of the earthing system —for reasons of equipment protection—, as well as distribution of potentials on the earth surface —for reasons of human security— when fault conditions occur. While very crude approximations were available in the sixties, several methods have been proposed in the last three decades, most of them on the basis of practice and intuitive ideas. Although these techniques represented a significant improvement in the area of grounding analysis, a number of problems, such as the computational requirements or the error uncertainty, were reported. Recently, the authors have identified these widespread intuitive methods as particular cases of a general Boundary Element numerical approach. Furthermore, starting from this BE formulation it has been possible to develop others more efficient and accurate. The Boundary Element formulations derived up to this moment are based on the hypothesis —widely assumed in most of the practical techniques and procedures— that the soil can be considered homogeneous and isotropic. A more general BE approach for the numerical analysis of substation grounding systems in nonuniform soils is presented in this paper. The formulation is specially derived for two-layer soil models, widely considered as adequate for most practical cases. The feasibility of this BEM approach is demonstrated by solving two real application problems, in which accurate results for the equivalent resistance and the potential distribution on the ground surface are obtained with acceptable computing requirements.