An efficient technique for determining contact voltages and grounding resistances in multi-stratum land, for standardised transforming centres

As a preliminary step before design of the grounding installation for a transforming centre, the terrain is surveyed by means of vertical electric sounding, usually using F. Wenner´s method. When the results are interpreted in order to predict stratigraphy, it is normal to find that the model corresponds to that of a multi-stratum terrain. In general, a minimum of two and a maximum of six strata are found. Very rarely, the terrain is modellable as homogenous. However, as is well known, there is at present no efficient procedure which enables the contact voltage and grounding diffusion resistance to be obtained quickly and precisely for electrodes with a complex geometry in the case of multi-stratum terrain types. As part of its new standardisation plan for transforming centres of up to 20 kV, 2×630 KVA (MTDYC Standards), and their corresponding grounding points, Iberdrola has developed a procedure to determine, with extraordinary efficiency, the contact voltage and grounding resistance in all standardised transforming centres, whatever the structure of the terrain (two to six strata) where they are to be located. The technique is applicable to grounding points for transforming centres of the following types: all-weather post; all-weather compact; surface prefabricated; and underground. The purpose of this paper is to present the procedure used, based on a technique of reduction inspired by the equipotential surfaces method, combined with an adjustment technique which uses the function χ², and utilises exponential, parabolic hyperbolic and where necessary, linear functions.