Reduction in Step & touch potential 1

[tin2779]

Hi,

I am not sure, if I am using a smart approach while trying to accomodate step and touch potential as it does not seems to be economical.

My GPR is high approx. 18 KV. but local authority wants us to satisfy Step and touch potential issues. I have reduced the size of meshes to half, I gues they have reduced from
6ft by 6ft to 1.5 ftby 1.5ft in a 80 ft by 38 ft area.

Just wondering, is there any other way to deal witha situation like this.

I would appreciate a word.

Thanks

 

[jghrist]

It is sometimes very difficult to get adequate step- and touch-potentials in small stations. Make sure that you consider only the current returning through the earth and not the total fault current. For small grids with relatively high resistances, most of the fault current may return through neutrals and/or shield wires.

What is the system voltage? You can't get more GPR than the line-to-ground voltage of the system. You can get enough fault current such that the fault current times the grid resistance will exceed line-to-ground voltage, but if you do, this means that less than the total fault current will be flowing through the earth.

 

[tin2779]

Thanks Jghrist. The voltage is 44 KV that means L-N is 27 KV so this does not seems to be the issue.My GPR is with in theoretical limit. The local authority has completely rejected the idea of considering sky wire as a return path and is only considering a self sufficient grid to satisfy step and touch potential issues and a remote grid to satisfy GPR requirement.

I managed both but just thought, is there any other way get these issues resolved in small substations ?

One more point, you can shed some light on:

1. I am getting a bit increase in step potential by reducing the size of meshes. Is this normal ?

2. While designing the grid, I am not considering any concrete structure foundation modeling. That means during a fault situation my touch and step on the swgr building could be an issue. I will be connecting rebars to the grid but below the foundation there will only granular material and no grid.
From your experience, concrete foundation of structures can pose a problem?

Regards,

 

[jghrist]

As you reduce the size of the mesh, you reach a point of diminishing returns. 1.5 ft by 1.5 ft is certainly at that point. You won't get much better with a solid plate. The soil surface above the grid will still be at a lower potential than the grid because there is a voltage drop through the soil vertically above the grid. You might try reducing the depth of the grid.

It is not really reasonable to assume that all of the fault current goes through the soil. If this were really the case, say if there were no sky wire, the fault current would be lower because the zero-sequence impedance would be higher. I don't know how you can convince the local authority of this. Maybe you can convince him by referencing IEEE std 80-2000 Chapter 15.

Another option would be to include the effect of substation grid resistance in the calculation of fault current. See IEEE std 80-2000, Section 15.4. I don't know how this could be accomplished if you were given the available fault current or if you calculated it with power system analysis software.

The concrete foundation should not be a problem, but if it is a slab foundation as opposed to deep piers, and is poured on relatively high resistivity granular material, it won't help much either. The foundation with its bonded rebar will all be essentially at the potential of the grid. You have to maintain low enough touch-potentials around the building perimeter to allow standing outside the building and touching the building. Inside the building, everything will be pretty close to the grid potential, so touch- and step-voltages will not be a problem. You could model the rebar and concrete in detail to prove this, but I personally do not think it is necessary. There could be a problem with transferred potentials if you have communication cables in the building. This is a GPR problem, however, not a step- and touch-potential problem, and could be solved with proper isolation.

 

[tin2779]

Thanks Jghrist. Your comments are like a picture and worth thousand words.

I used WinIGs and my fault current is after considering a power flow analysis option which considers grid impendance as the fault impedance. My fault impedance is 24 ohms.

I guess you cleared most of my doubts. I am just wondering, if I have to theoretically picture increasing the depth of grid, how it can effect the step and touch potential ?

If you could just drop few words, that would be helpful.

Thanks

yes one more things Jghrist, I am not sure, if you recall but an year ago, I used to have a different username. You answered all my questions on ground grid and today if I could design ground grids. I owe it to you for this expertise. So thanks.

 

[jghrist]

Annex F of IEEE Std 80, Section F1.3, discusses the effect of grid burial depth on step and touch voltages. There are also graphs of step and touch voltage vs grid depth. Up to a point, increasing grid depth modestly reduces touch voltage. Beyond this point, increasing the depth increases touch voltages. This point is lower for more dense meshes. It comes about because of the vertical component of the voltage drop in the soil.

Try varying the grid depth in your WinIGS model.

Some of the current from the grid goes up and then out. This causes a vertical voltage drop. With large meshes, increasing the depth will flatten out the voltage profile, and the voltage at the middle of the mesh will be closer to that directly above the wire. Even though the voltage drop directly above the wire will be lower for shallower grids, this is offset by the increased flattening of the voltage profile. With small meshes, the voltage above the mesh is pretty uniform anyway even with shallow grids, so the vertical component of the voltage drop will be more important.

Consider an extreme case of meshes smaller than your foot and at the surface. In this case, there will be no touch voltage; you are effectively covering the entire substation with a switch operating platform or mesh. Your feet will be at the same voltage as the grid (because you will be standing on the grid) and the equipment you can touch will be at the same voltage at the grid because it is bonded to it.

 

[7anoter4]

Hi tin2779,
In my opinion the IEEE80 is not universal applicable.
Other theory as Optimum Design is more suitable for your application:
"Optimum design of substation grounding in a two layer earth structure"
By F. DAWALIBI & D.MUKHEDKAR-MONTREAL CANADA 1974
Also using 10-20 vertical grounding rods of 3/4" diameter and 20-50 feet long
may reduce the grounding grid resistance .
The material supplied by Erico

http://www.erico.com/products/GEM.asp

ERICO's Ground Enhancing Material (GEM) is a superior conductive material
that improves grounding effectiveness, especially in areas of poor conductivity
such as rocky ground, areas of moisture variation, and sandy soils.
I hope this may help
Regards

 

[jghrist]

The use of ground rods will help in tin2779's situation more than normal because of the local authority's disallowance of using current division. Normally, ground rod effectiveness in reducing touch voltages is reduced because as the grid resistance decreases, more of the fault current flows through the earth.