Soil resistivity for a 138kV substation

[Madman4q]

Hello all,

What kind of soil resistivity test is used to obtain soil resistivity values to be used to design a ground grid for a 138kV substation? I see the 4 point wenner method is used and also lab resistivity testing with the soil sample.
Which method is generally used by utilities or others for substation ground grid design.
Also, there's a peaking power plant about 100ft away with a different owner and the ground grids will NOT be tied together. Will this affect the results of any of the soil resistivity tests?
Thanks in Advance!

 

[jghrist]

The 4 point Wenner method is the most commonly used resistivity test. I gives values of resistivity at different depths, which is important for ground grid design. A lab resistivity test only gives the resistivity for the sample taken.

Resistivity at large depths is required for large grids. Measuring the resistivity at large depths requires large probe spacings, and the grid 100 ft may affect the results.

See 81-1983
IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System.

 

[Madman4q]

jghrist thanks for your comment. I'll look this up.

 

[cuky2000]

There are several acceptable methods to test the soil resistivity. The 4 points Wenner method is one of them.

Most meters today have filters to isolate electrical noise.

The effect of an energized facility 1000' apart may not introduce significant distortion in the test.

Soil resistivity measured in the lab from sample, many time are not representative of field conditions.

Regardless the method used to test the soil resistivity, the measured apparent resistivity require interpretation to obtain a equivalent soil model (uniform or multiple layers) by the Sunde's method, least square, etc.

See the enclosed link for additional reference.

http://www.lyncole.com/pdf/Lyncole_Ground_Test_Methods.pdf

 

[zinkblind]

Dear Madman4q

Attached you well find a pdf file showing the commonly used methods in soil resistivity measurement.

If you need any thing about that do not hesitate to tell me.
i.e. grounding design


Best regards
Ahmad