Geoelectric modeling for the design of grounding systems - the determination of the required model depth

Abstract:

Wide-area grounding systems, such as windfarms and big solar plants, have dimensions of kilometers. However, in the majority of the projects, the grounding systems of these generating plants are still designed similarly as it is done for energy substations, using shallow geoelectric models obtained from electrical surveys, using AC and low power ground meters, with the Wenner arrangement and current probes spacing limited to 100 m.

The problem here proposed is the determination of the required depth of the geoelectric model to be used for the grounding system design, which shall be compatible with the topology and area of the grounding system and with the tectonic setting of the plant site. Sites located within sedimentary basins will require shallower geoelectric models, while those located within cratonic areas will require deeper geoelectric models. Once the model depth is determined, the ground survey can be specified, usually including a combination of shallow vertical electrical soundings (VES) with audio-magnetotelluric (AMT/MT) and/or time-domain electromagnetic (TDEM) surveys.

Three layers’ 1D geoelectric models are suggested for three basic tectonic settings, considering three eons – “Archean” (> 2.5 Ba), “Proterozoic” (> 541 Ma) and “Phanerozoic” (< 541 Ma), aiming the evaluation of how the regional tectonic setting interferes on the required depth of the geoelectric model. The proposed methodology is based in the development of a parametric study to evaluate the sensitivity of the grounding resistance, which is a basic parameter of any grounding system, to the combination of different tectonic settings, depths of the crystalline basement and grounding system dimensions.

This multidisciplinary paper applies electrical engineering and geophysics knowledge for the solution of an engineering problem, with a pioneering approach for the determination of a deep geoelectric model for the design of a wide-area installation grounding system. It is beyond the scope of this work the presentation of the methodology for the shallow and near-surface data processing, including the correction of the static-shift deviations, aiming the construction of the combined geoelectric model, from the soil surface down to the required depth.