Ground grid design-limiting touch voltage

[azadani]

we are invoved in designing ground grid for a 63/20 Kv sub.(area: 120*80 sqm).The fence can not be included within ground grid area.the outer ground wire is 1 m inside the fence. our reccomendation is to isolate the fence from the grid and earth it seperatly but our client wants to connect the fence to the grid.
This induce a serious problem in design.we can not limit the touch voltage below tolorable voltage 1 meter outside the fence even with 2 m spaced ground rods along the grid perimeter.we would like to have your reccomendation on design and/or to justify the client for another design criteria.
I thank you in advance.

 

[Marmite]

You could try running an additional perimeter ground conductor 1m outside the perimeter fence, and bond that and the fence to the main substation ground. How close is the fence to the nearest item of plant? If it's less than 2m then you will have problems with your proposed solution if you ground the fence separately as there will be touch potential between the fence and the plant.
Regards
Marmite

 

[jghrist]

If the fence is not bonded to the grid, then there will be a touch potential problem for someone standing inside the fence over the ground grid and touching the fence.

A perimeter wire outside the fence bonded to the fence and the grid is the solution, but physically this is not much different than extending the grid outside the fence. Neither is putting ground rods every 2 m along the fence. Why can't you extend the ground grid outside the fence? If it is property problem, maybe you could put the fence 1 m inside the property line.

 

[WindTurbinesAreFine]

Do you have overhead lines coming into the substation, or is it underground cable?

If you have overhead lines crossing over your fence, you are going to have big problems if that energized conductor falls on the ungrounded fence.

The problem for you I suppose is that in order to limit the touch voltages, you would need to expand the surface area covered by your grounding grid, and that sounds like it's not an option for you.

 

[azadani]

I would like to draw your attension to following points.
1-Due to property problems with substation neighbours in 3 sides ,we can,t extend the grid outside the fence.
2-As a fence there are actualy prefabricated concrete walls with 2.5 m height around the sub. but some mesh type metalic guards have been erected on the top of the walls.
3-There isn,t any item or let say structure near the fence.
4-For someone standing inside the fence and touching fence ,since there is no direct connection to the main grid,
i don,t think there would be any danger .
5 There is overhead transmission line comming inside the sub. over the fence.but i can,t realize why we should expect major problem for falling energized line over the seperately earthed fence.Like the similar faults near the sub. ie falling line on metalic substances ,the distance relay senses the fault and send trip command .
I thank you for some more clarifications on items 4 and 5 as stated by my colleagues in this forum.
Thanks

 

[jghrist]

Let's say hypothetically, that a 63 kV fault in the substation would raise the grid voltage to 2 kV above remote earth potential, and the earth surface potential directly above the grid is 1.9 kV above remote earth. Also, hypothetically, the earth potential drops off around the grid such that the potential on the fence is only 1 kV above remote earth during the fault. Then someone standing above the grid and touching the fence would experience a 900 volt (1.9 kV - 1 kV) touch voltage.

It is also possible that the earth surface potential outside the fence is lower than the fence potential enough to present a touch potential danger outside the fence.

Your recommendation might be safe, but not necessarily.

 

[WindTurbinesAreFine]

Have you run any simulations yet with your preliminary design to see what you're up against? In general, what does your earth resistivity look like, <100 ohm-m, >100ohm-m?