Soil electrical resistivity measurements for a wind turbine location

[Cesme]

Hello everyone, I wonder if 2 90 degree cross measurements (traverses) are sufficient in the wind turbine field? According to BN 50522, spans (a = 1 m - 1.5 m - 2m - 3m - 4.5m - 6m - 9m - 13.5m) are recommended as amax = 13.5 m. Does anyone have a different comment or advice on this subject?

 

[jghrist]

I would suggest 2 cross measurements for each wind turbine. The maximum probe spacings depend on the diagonal dimension of the turbine ground grid. See IEEE std 81, Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System, for guidance.

 

[Cesme]

Thank you for your response. To be honest, when I look at the literature, there doesn’t seem to be a universally agreed-upon number of traverses. However, it is generally recommended to perform at least two measurements. Previously, four measurements were conducted at the turbine site, and initially, I also stated that four measurements would be sufficient. However, due to the increased cost, I reduced it to two measurements. The client is now skeptical about whether two measurements would be adequate. The lack of a clear standard for the number of measurements is frustrating. While increasing the number of measurements provides more data, it also raises the cost for the client. Unfortunately, I don’t have a definitive source to substantiate the recommended number of traverses.

 

[jghrist]

A lot depends on how variable the soil resistivity is. Of course, that's hard to tell without making several measurements. Variation with depth can only be determined with long traverses. It also depends on critical the resistivity is. I'm not familiar with grounding wind turbines, but unless there are facilities other that the structure itself that can be touched, there may not be a need for more than a simple perimeter ground wire a meter from the structure to prevent dangerous touch voltages. I assume that the foundation will be very large, either very deep or large diameter. Tying the rebar to the ground wire (Ufer ground) would provide a low resistance ground. If the foundation is very wide, like a large reinforced concrete disk, then it will be an equipotential surface around the turbine structure and virtually eliminate touch voltage concerns.

IEEE std 81 has a good discussion on how many and how long traverses should be. There is no definitive answer.