Grounding Design with Very High GPR

[asadurrehman]

Hello
I am handling the design of a substation limit whose grounding design is with with very high GPR, while Step and touch are within limits. The normal practice is to use conductor and rods.
What possible method is to bring GPR close to the acceptable limit.
Can I use remote contribution less than 100%.
The surface thickness more than 1.5m.
The asphalt surface resistivity more than 3000 ohm m?
The number of outgoing feeders more than 1 and distribution feeders as per actual?
The utilization of GEM in extremly dry rocky area?

The available area is 50x50 m and top layer is having resistivity more than 1000 ohm m.

Please guide me in this regard.

Asad

 

[jghrist]

What is the need to limit GPR? If there are no transferred potential problems (remote ground level being accessible from elevated earth potential), then there could be a high GPR without a safety problem if step- and touch-potentials are within limits.

You can use remote contribution less than 100% only if it is less than 100%. Is part of the ground fault source local?
Adding surface thickness or asphalt will increase allowable step- and touch-potential but will not lower GPR.
Outgoing feeders with grounded neutrals will reduce the amount of current flowing into the grid and the GPR. Some of the fault current will flow through the neutrals and return to the source through the neutral grounds. If the resistivity is high, this may cause touch-potential problems at the distribution feeder ground points (transferred potential).
Using GEM can reduce GPR.

 

[7anoter4]

I agree with jghrist: if you have not potential transfer problem you don't need to reduce the GPR if touch and step potential are in permissible limits.
However, if you intend to reduce the GPR you have to reduce the grounding resistance.
You may try the soil treatment with material like this one:

http://www.saraswateng.com/TEREC_Data Sheet.pdf

 

[Power0020]

Are there any outgoing power cables? this is usually the one thing to worry about for limiting GPR. Some local standards ask for 5 kV limit.

Check also if any other cables are bonded to the grid.Communcation cables may need to be isolated in the substation to avoid transferred potential.

very important to check if this substation is within an inhabited neighbourhood,also, check communication cables are not interfering with GPR of 430 V and 650 V contours as per IEEE 80.

This article should give you an idea about risks.

https://en.wikipedia.org/wiki/Earth_potential_rise

 

[asadurrehman]

Thanks a lot for your responses. We have FO cable for automation and communication purposes. The normal practice is to keep it close to 5000V. However, we have a situation that adding more conductor and rods is not having any considerable impact on GPR. The reinforcement of concrete , piping and all other equipment are grounded in the system.
Is GEM a reliable method in dry rocky area?

 

[bacon4life]

We used grounding wells more than 70 meters deep to lower GPR at a couple of our substations in gravelly/rocky areas.