Geoelectric modeling for the design of the grounding system of the Bom Jardim da Serra windfarm

The Bom Jardim da Serra windfarm is situated on the southeast edge of the Santa Catarina Plateau, border of the Serra Geral Formation, 1400 m above sea level. This plateau is bordered by huge and beautiful canyons that are very close to some aerogenerators and was produced by the extensive fissural volcanism that 130 million years ago flooded 75% of the entire Paraná Basin, South of Brazil, with 800,000 km³ of lava. These geologic events formed volcanic layers with an average thickness of about 800 m, but also up to 1500 m in the axis of the Paraná Basin.

The windfarm of this study comprises 62 aerogenerators and its grounding system is all interconnected, occupying an area of about 28 km². Moisture is very high in the area and the basalt basement is covered by a less than one-meter thick layer of organic soil (black peat). There are basalt outcrops everywhere in this area and they are well-intact in some places, in others quite altered.

The grounding simulation software requires a 1D geoelectric model, which was constructed based on the data acquired by a comprehensive geophysical survey, with 22 vertical electrical resistivity soundings, using Wenner arrays, and 22 audio-magnetotelluric (AMT) recordings at the same locations. The survey also included the measurement of the grounding impedance of 22 towers, with a high-frequency ground resistance tester (25 kHz).

This is a multidisciplinary paper that applies electrical engineering and geophysics knowledge for the solution of an engineering problem. The paper will present the methodology used for obtaining the 1D geoelectric models for three areas of the windfarm, from the combination of the shallow and near-surface geoelectric models produced from the measured data. The static-shift of the average Wenner apparent resistivity curves (3 curves, one for each area of the windfarm) was constrained by the measured towers' impedances. The static-shift of the averaged 1D AMT apparent resistivity curve was constrained by the expected depth of the Serra Geral Formation, which is about 700 m at the border of the canyons.

The adjusted 1D geoelectric model shows that in the area of the windfarm, the volcanic package is superimposed over a layer of pre-volcanic and low resistivity sediments. Despite the basalt basement that dominates the entire area, the geoelectric models show lower resistivities than expected, which indicate that the basalts in the area are well-fractured and water-saturated. The 1D geoelectric models are compatible with the expected geological structure of the border of the canyons and were used for the simulation of the towers' groundings, allowing for the reproduction of the average of the measured towers' impedances.

In this paper we discuss a pioneering approach for the design of the grounding system of a windfarm, applying extensive geoelectric modeling, compared to the usual grounding design that is based on simple shallow geoelectric models, developed from Wenner soundings, with current probes openings limited to about 100 m and with no static-shift adjustment.