GPR study after grid installed

[tricard]

Hey all,

Been a while since I have posted or been on this forum.

I was contacted by a contractor who has already installed grounding infrastructure for an islanded 600/347V system supplied by a 2MW generator. The island system has a section of 27.6kV overhead line to reach the far side of the property, which is stepped up through a 600/347 - 27.6/16kV transformer. The AHJ is now looking for a GPR study of the system, but of course the contractor has already built this system so he is kind of panicking since he doesn't have a study in hand (thus he contacted a nerd ...me... to get him this).

My previous design of ground grids has always been "green field" and would use the soil resistivity and substation plan as input data; design the grid, calculate GPR, redesign as needed then verify through measurements that the installed grid meets the calculated grid resistance from the study. I've never worked backwards from that... I can get the grid resistance but getting soil resistivity at this point won't be possible since the grid is already there. I doubt any drawings of what was installed exist, so I don't know how I model what was installed... or if I really have to. Am I overthinking this whole thing? Should I just calculate the grid current based on Sf assumptions from IEEE 80 then use the current and measured resistance to determine GPR. That seems oversimplified though.

What is my feeble mind missing in this!

Thanks for the help.

Tim

 

[Kiribanda]

Knowing the maximum GF current at the location, you can measure & calculate the STEP, TOUCH potentials & GPR
using 4-pin earth tester available in the market.

 

[7anoter4]

Since the source is located on the ground grid no GPR is expected on the grounding grid around the generator.
The short-circuit at the far location returns to the source through ground and will produce GPR on both grounding grids.
What Kiribanda proposes [in my opinion] it is to produce a current injection to the far side through the ground and measure the touch and step potentials on both side. [In the old IEEE 80 is an example of how in the old times they did the measurements].
The short-circuit has to be done using a source [a welding transformer for instance] located close to generator through overhead line neutral and return through grounding grids.
The actual touch potential will be proportional with actual short-circuit current.