Layered Crust Resistivity Model for Windfarm Grounding Studies
Abstract
This work presents the development of layered crust models for the study of the grounding system of Agua Doce Windfarm Complex, localized north of Santa Catarina State, Brazil. This complex has 6 windfarms with a total of 86 aerogenerators, distributed by an area of about 220 km2. Agua Doce Complex is located at the top border of Santa Catarina state, directly over the basalts of Serra Geral Formation, upper layer of the Parana Sedimentar Basin, which lays over the huge confined Guarani Aquifer, formed by varied sediments accumulated over 250 my (from Ordovician to Triassic Period). Below Agua Doce, the Serra Geral Formation is around 1km deep, being the Parana crystalline basement about 3.6 km deep. Windfarms are extensive power plants, occupying wide areas. The grounding of each tower is composed by the huge amount of steel rebars, inside the tower foundations, and complemented by buried cooper cable rings. The tower groundings are interconnected by means of buried cooper cable or by aerial steel cables, the latter fixed on top of the distribution lines that connect the aerogenerators to the main substation. The steel cables of the aerial lines are also grounded by means of spaced grounding rods. For the simulation of this wide grounding system, it is essential the previous development of a layered electrical resistivity crust model. This model will be dependent on the quantity and quality of resistivity measurements, and also on the adopted technique to reduce the large amount of measured values to an average apparent resistivity curve, which shall be representative of the initial data set. The desired crust model should represent the different resistivity values for at least 10 km deep, including a detailed representation of near-surface soil layers, down to 100 meters, considering that this is the medium where the ground electrodes will be buried. For this project, soil resistivity measurements were made close to each of the 86 aerogenerators, by means of Schlumberger arrangement, with maximum spacing between current probes from 0,4 to 1km. The measurements show that the shallow layers in general present the same trend, considering the shape of the apparent resistivity curve, but with significant resistivity values variation, what was expected, considering the wide area surveyed. For complementing these near-surface measurements, it was included a deep crust model obtained from seismic and magnetotelluric measurements, made 115 km away from the windfarm. The resistivity models were developed based on equivalent apparent resistivity curves, obtained from different sets of resistivity measurements. We present a comparison between the equivalent curves obtained by means of simple geometric mean with curves developed by the application of nonlinear optimization methods (Marquardt, Simplex or Steepest), based on the least squares method. Two kinds of resistivity models are presented: - wide area models - applicable to each of the 6 windfarms; and - local area models - applicable to small clusters of aerogenerators, that are close to each other and do not have too long grounding interconnections.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMNS33A1673F
- Keywords:
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- 0900 EXPLORATION GEOPHYSICS;
- 0925 EXPLORATION GEOPHYSICS Magnetic and electrical methods;
- 9360 GEOGRAPHIC LOCATION South America