Expanding Substation Grounding System

[rockman7892]

I'm looking at a project for expanding an existing 34.5kV Substation. Existing substation is a fenced in yard that has 34.5kV overhead feeders that transition to underground to feed 34.5kV Switchgear in prefab control building yard. The existing yard has 34.5kV - 4.16kV transformers with 4.16kV low side feeing switchgear lineup in same building.

Existing yard has crushed rock. Existing Substation grounding has a perimeter 4/0 ground around perimeter of yard and is tied to perimeter fence and is crisscrossed across yard to tie into specific equipment locations. I cant tell if grounding design was based on a ground grid study with step/touch cacls per IEEE or if grounding was based on some other "standardized approach"

Customer is not looking to expand the yard to put another control building which has switchgear fed from a parallel tap off of the same overhead 34.5kV utility lines. New switchgear will provide a redundant feed to existing transformers.

My question is with the expansion of the existing yard to install a new control house is it required to perform a ground grid calc and analysis per IEEE or would this fall under more of a "standard approach" with expanding existing yard grounding to just encompass the perimeter of the new control building to connect new building to existing substation grounding?

 

[Power0020]

I'd expect a site current injection testing to confirm the existing grid safety and then a revised study for many reasons:
1- confirm the existing design matches testing.
2- confirm the proposed design is safe with upgraded fault and clearance times, if any.
3- as the substation grid area increases, transferred voltage hazard zone outside the substation will also increase and may include third party utilities (pipelines, LV networks, telecom...etc).

 

[DBL-E]

Just wanted to affirm Power0020 to affirm current system is compliant, and progress to a new design.

Typical reasons for a grounding study at existing sites are:
-Maintenance and field measurements find deficiencies
-System protection has changed
-Power system short circuit availability changes (Fault current levels now may be greater than from the original design)
-Site arrangement modifications (simply moving a fence can take site from compliant with IEEE 80 to non-compliant)

If they don't have the documentation I would assume the grounding system was not studied and should be evaluated as part of the system updates. One benefit of expanding the ground grid is you should lower the impedance of the existing grid, but you don't know if that results in compliance to IEEE or not.

 

[jghrist]

The expanded ground grid must comply with standards and provide safe touch- and step-voltages. You could make the mesh sizes no larger than the existing ground grid, but unless you model and analyze the grid, you cannot assume that the existing mesh sizes provide safe voltages.

 

[cranky108]

I believe ground grid design requirements might have changed over the years, so a new study is not out of place. Ground grid sizing depends on maximum fault current, ground resistance, and amount of crushed rock. And seeing how crushed rock has a specified life, you may need to add rock.
None the less, I think a new building would require modifications of the existing grid, and if a new transformer is being added, a new oil containment plan.

 

[Fischstabchen]

Maybe, a better question to ask would be "how comfortable would you be if you were brought into court due to field tech. being killed due to your ground grid design being non-compliant with the requirements set forth in IEEE-80?"