Design ground substation on a rocky terrain 2

[slugz]

Hi Guys,

My client want to install a substation on a mountain literally, there is a big rock underground all over the terrain, we can`t dig and explode anything (ambiental)...

In these cases how can i do equipotentialization?? any sugestion?

Terrain have 25x30 meters
Isc = 14kA
Two transformers...

I dont know if i am correct, but in this time, what blow in my mind is only put one or two meters of a good soil, to perform a small small reticulos mesh in order to provide a equipotentialization..(very expensive cost but is only what i think right now).

Anyone had this problem, how solve?

*Sorry for my english

Thanks

 

[davidbeach]

Station in what it is. Ground grid design is probably "too much" copper "too close" to the surface.

 

[bacon4life]

Drilling grounding wells? Worked in our gravel pit, but not sure about bedrock.

 

[IrishPower]

slugz,

If you expect a high ground grid resistance, bear in mind that this may reduce / restrict the ground fault current to less than the 14 kA value you mentioned. This won't solve all of your problems, but if your utility allows you to do this it may knock a few percent off your calculated Ground Potential Rise and touch / step voltages. Also, the proportion of your total earth fault current that comes from circulating currents in any solidly-grounded transformer neutrals at the substation may not cause GPR in the event of a ground fault at the substation. Again, this might only knock a few percent off the GPR if your utility allows you to take this effect into account (and if they provide you with a detailed breakdown of where the 14 kA comes from).

The only obvious ways to significantly decrease the earth grid resistance in your case are if the soil structure (and resistivity) show that deep earth rods would be of benefit (unlikely in your case?), or if you have the possibility of extending the size of the earthing system by laying counterpoise (hozizontal buried earth conductor) in cable trenches leading away from the substation, and connecting the counterpoise to the earth grid. This may or may not be possible for your site, or allowable by your utility, but it may be worth checking...

Hope this helps.

IP

 

[HamburgerHelper]

In Argentina, someone told me they go cheap on the grid but make the workers wear rubber boots. I don't know if you can get away with that elsewhere but it probably is a cheap solution when you can't get or afford a good grounding grid.

 

[Mbrooke]

I think people are confusing grounding with bonding. Grounding has nothing to do with reducing step potential unlike bonding. A properly deigned ground grid will take care of voltage differences inside the substation regardless of soil conditions.

 

[slugz]

Thank you Guys for all support. At now, i'm doing some simulations with CymGRD software, and we will try use a product called GEM, we are in contact with support factory. Let see if we'll found a good solutions $$$ in this case.

i will put a resolution of the facts here..

 

[cuky2000]

Below are a few suggestions for your consideration:

1) Place the ground grid 18" to 24" below the finished grade with the adequate calculated separation.
2) Excavate or cover the soil with suitable material with low resistivity and structurally acceptable.
3) Cover the top of the backfilled soil with 4" to 6" granular clear with high wet resistance surfacing material (Typ. #2 crushed rocks)
3) If the site water table is near 20 ft. to 30ft., make multiple wells core drilling the rocky soil with ~4" bit driven by rotary drilling machine
4) Insert in the 4" hole a long ground rod (use multiple 10' rods extended with couplers).
5) Place the rods in the center of the 4' wells and fill them with Portland cement, of a suitable hygroscopic filling material.
6) If necessary, run a continuous grounding conductor UG or OH to a low resistivity location such valley near water body and built a ground electrode.
7) Take advantage to ground properly the substation foundation and use them in the grounding model software.
8) Use proven software to model the grounding system or hire a qualified consultant to perform this work.

Good luck