A formula for resistance of substation grounding grid in two-layer soil

A simple and accurate formula for calculating the resistance of a grounding grid buried in two-layer soil, is proposed in this paper. The formula is derived from the image theory and a novel synthetic asymptote technique. The resulting formula is dependent on both the substation grounding grid design parameters (such as grid size, number of meshes, grid-conductor diameter, grid depth, etc.) and the soil parameters (such as the resistivities of the upper and lower soils). The resistances calculated using the proposed formula are compared to the resistances obtained from a detailed numerical method and the agreement between the results is very good.<>