Mate Kurtović

NUMERICAL ANALYSIS OF EARTHING GRIDS BURIED IN HORIZONTALLY STRATIFIED MULTILAYER EARTH

This paper describes a new numerical procedure for analysing earthing grids buried in horizontally stratified multilayer earth. The procedure is very efficient and general. The total number of layers and the total number of metallically disconnected earthing grids are completely arbitrary. A single earthing grid can be positioned in several layers. The procedure is based on an integral equation formulation. Earthing grid conductors are subdivided into segments and the average potential method is used. Efficiency and generality of the computation procedure are based on the successful application of numerical approximations of two kernel functions of the integral expression for the potential distribution within a single layer which is caused by a point current source. Each kernel function of the observed layer is approximated using a linear combination of 15 exponential functions. Extension from the point source to a segment of the earthing grid conductor is done by integrating the potential contribution due to a line of point current sources along the segment axis. This computational procedure gives highly accurate results in a short execution time.

Statistical analysis and interpretation of resistivity sounding data in earthing grid design

This paper presents a statistical analysis and interpretation of resistivity sounding data, with the objective of establishing an optimal earth model for rocky soil. The parameters obtained are used as input data for a CAD of the earthing grids of an electric power substation. In this analysis, the soil is represented as several vertical zones, each of which is modelled as a set of horizontal layers. Field observations are carried out at each sounding point using general configurations of four symmetrical electrodes; the Schlumberger and Wenner configurations are special cases of these. Apparent resistivity is computed from the field measurements using a new expression which takes into account the cylindrical form of the electrodes. Sample values of the resistivity transform are then computed from the field apparent resistivity data using an exponential approximation of Stefanescu kernel function. The observed soil is subdivided into zones using a statistical analysis of the field resistivity transform data. Parameters for the horizontally stratified earth model are obtained by an iterative interpretation of the average resistivity transform of a single zone. During the iterative interpretation, the sample values of the average resistivity transform computed from field observation data are compared with those computed from the layer model parameters. The comparisons are efficient, since they use a variant of the Marquardt method.