Addition to Earlier Post 4

[SilverArc]

Hi Everybody,
I had sneaked in an earlier post"Grounding in Rocky areas" and Jghrist was kind enough to answer my question there about the potential variation shown by dotted lines in fig. 1 found in the link below:

http://www.era.co.uk/Docs/ECS/Ground_ROD_Tutorial.pdf#search="touch voltage step potential + grid"

Hi Jghrist,
Thanks for these valuable comments. We dont have any body at our work, who has even a figment of imagination about grounding. So, I try to work out concepts on my own or some times people in this forum are kind enough to advise me.
After reading through explanation,
"The potential shown by the dashed lines is not the potential beneath the earth. This is the surface potential as it varies along a line on the surface and it is shown below the surface for convenience"
we have a uniform copper mesh below the substation, where an equal amount of current will flow, So The potential should be same also ?
I thought through it and came up with explanation. The variation in potential on the grid its self is depending on the fault happens. Is this the reason ?

Q# Why do we say the "Mesh potential"(the potential in the centre of the grid) is always maximum.

Q# Eventually to give more practical shape to my dilemna, I did a simulation in ETAP and came across the plot below for touch profile.

http://img245.imageshack.us/my.php?image=gpr1do8.jpg

Could you please explain me, what are these red circles at different places. What do they highlight. I though a lot about it but could not get it.

Thanks.

 

[jghrist]

The copper mesh may be uniform, but it is not solid. The surface potential in the middle of the mesh is different than that directly above the wire. The section cut in the first link is through the middle of one row of meshes.

The grid wires themselves are usually considered to be at the same potential, with no significant voltage drop. The variation is in the soil surface voltage.

Touch potential increases in the middle of the mesh, but the surface potential decreases. Touch potential is the difference between the surface voltage and the grid voltage. Equipment and steel is connected to the grid and is at the same voltage.

The green surface on the ETAP plot is touch potential, not surface potential. The red circles indicate a contour of the touch potential on the surface. The GPR is not shown, but assume for example that it is 2000 volts. The voltage scale on the left is the difference between the grid voltage of 2000 volts and the surface voltage. In the middle of the corner meshes, this difference is the highest. The surface voltage is 1600 volts. The difference between the grid voltage (2000 volts) and the surface voltage is the touch potential, 400 volts. This high touch potential area is indicated by yellow spots on the surface contour plot. Above the grid wire crossings, the surface voltage is higher, peaking at about 1900 volts, and the touch potential reaches a minimum of 2000 - 1900 = 100 volts. This is shown by the green diamond shapes on the surface plot. The red circles are areas where the touch potential is between 200 and 300 volts. Still assuming a grid voltage (GPR) of 2000 volts, this would mean the surface voltage where the red circles are will be between 1700 and 1800 volts.

 

[SilverArc]

Hi Jghrist,

Thanks for your reply. I took the weekend to understand and evaluvate my concept about ground potential rise.

I got most of it but a few points just kept bugging me. I guess few more moments of your time will be helpful.

"On a conceptual basis, In such a big grid and it is all uniform: So why in you post why do you different the term MESH and SURFACE". Like you said
"Touch potential increases in the middle of the mesh, but the surface potential decreases"

I am just trying to make an effort to get this thing to in to my head.
Thanka a lot again for your earlier posts.

Regards,

 

[jghrist]

By "mesh", I mean an individual rectangle within the ground grid which is made up of crossed wires. The spacing of the grid wires (size of each mesh) is typically between 20 and 50 feet. By "surface", I mean the surface of the soil above the grid. The grid potential is nearly constant. There is some voltage drop in the wires, but usually negligible compared to the voltage drop in the soil. The surface potential is not constant. At points close to the grid wires (directly above the wires) the surface potential is closer to the grid potential. At corners, because the surface is closer to more wire, the surface potential is the closest to that of the wire.

The wire is at a higher potential than any point on the surface. The potential decreases within the soil as you get further from the wire. Points on the surface in the middle of a mesh have a lower potential than those directly above the wire. Points on the surface outside the grid have even a lower potential as you get further from the wire. After some distance away from the grid, the surface potential becomes almost the same as that of remote earth (zero as a reference voltage).

[tt]
The whole grid:
+------+------+------+------+------+------+
| | one | | | | |
| | mesh | | | | |
| | | | | | |
+------+------+------+------+------+------+
| | |another | | |
| | | mesh | | | |
| | | | | | |
+------+------+------+------+------+------+
| | | | | | |
| | | | | | |
| | | | | | |
+------+------+------+------+------+------+
| | | | | | |
| | | | | | |
| | | | | | |
+------+------+------+------+------+------+
[/tt]

 

[SilverArc]

Thanks Jghrist. I honestly appreciate your time. I have started doing simulations on ETAP and guess I am getting some results as per my understanding. I guess it will take time but eventually I will get it.

Just wondering, I have never seen a grounding grid during construction. How far is normally, grid under actual floor of the substation ? Is bringing the grid more closer to the floor will make the situation less miserable in terms of potential issues.

To be honest, I am completely ignorant to construction issues. I have ofcourse read that below the surface, Gravel or rock is added to increase resistance of the floor.

Best Regards,

 

[jghrist]

What do you mean by "floor"? Are you talking about an indoor substation? My discussions have been addressing an outdoor substation. An indoor substation will have a reinforced concrete floor which will be nearly an equipotential surface. Special attention is needed for the grounding of high voltage GIS because of the high current in the switchgear enclosure itself during a fault.

In an outdoor substation, the grid is normally installed around 18 inches (0.5 m) below the surface of the earth by trenching, laying the wire in the trench, connecting the crossing points and ground rods, and backfilling. Shallower or deeper grids will change the surface potentials, but this depends on the grid spacing and the soil resistivity. If the grid is too shallow, it causes construction problems when conduit or direct-buried cable is installed. You can vary the depth in your ETAP models to see the variation in touch- and step-potentials for a particular substation.

A 4-6 inch covering of crushed rock is then normally placed over the bare earth. Besides keeping things from getting muddy in the rain, the crushed rock insulates a worker's feet from the earth and reduces the amount of current that will flow through his body from hand to feet if he is touching a grounded object during a fault (from touch-potential) or flowing from one foot to the other (from step-potential). Using more than 6 inches of crushed rock makes it difficult to drive a truck around the substation without sinking into the crushed rock.

 

[SilverArc]

Thanks Jghrist.
This post of yours brought a new twist in to my thinking now about outdoor and indoor. I will get back to you after understanding the above comments and researching some information.
Again, I appreciate you time very much.
Best Regards,

 

[SilverArc]

Hi Jghrist,

A small hitch in my efforts, so I need a piece of advise from you. HOw do we abbreiviate GPR: Is it Grid Potential rise or ground Potential rise.

Why it bothered me was: It screwed my defination as what I understand from Ground potential rise is: The rise in potential of an equipment that is remotely located but connected to a line which is bonded far away to the grid where the fault has occured and that rise in potential compared to the local earth will be called Ground potential rise.

But the Grid potential rise is also same thing because
eventually that potential will be same as the potential of grid.

Could you kindly advise me, what I am missing here.

Thanks

 

[dpc]

"GPR" is generally considered to mean "ground potential rise". This is the value that the telephone company will be very interested in.

 

[jghrist]

Ground potential rise (GPR): The maximum electrical potential that a substation grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. This voltage, GPR, is equal to the maximum grid current times the grid resistance. (IEEE Std 80-2000 Definition 8.1.1)

 

[SilverArc]

Hi jghrist,

Thanks for your efforts. I guess I am good to go atleast not bad for a start for designing a grounding grid. I was reading one of your replies;

"In most substations, there isn't a lot of area where there is no equipment or plans for future equipment. I generally ensure that there are safe touch potentials throughout the fenced area and 3 feet outside the fenced area. This does not mean evenly spaced grids. Closer spacing around the perimeter and at corners is usually a more efficient use of copper because potentials will be higher around the perimeter if the grids are evenly spaced. Spacing in the center can be larger, but consideration has to be given to keeping the grid conductors close to equipment to reduce the length of ground stingers that connect the equipment and support structures to the grid"

I did not get this part "This does not mean evenly spaced grids. Closer spacing around the perimeter and at corners is usually a more efficient use of copper because potentials will be higher around the perimeter if the grids are evenly spaced. "

I am not sure but is this statement related to the concept that touch potential is more inside the substation and step potential is outside the substation. Kindly elaborate, if there is a concept associated with it.

Best Regards,

 

[Skogsgurra]

jghrist,

Excellent work! PLS for you.

SilverArc: there is a "Thank jghrist etc" to click if you value his work.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jghrist]

With evenly spaced grids, like the one I drew in my 16 October post, all of the meshes are the same size. The touch potential in the middle of each mesh will not be the same, however, because current densities are higher on the outside perimeter of the grid. To put it another way, more current flows into the earth from wires that are near the edge of the station than from wires near the center of the station. Touch potentials in the middle of a grids along the edge of the station will be higher than in the middle of grids in the center of the station.

Touch potential is also higher in the middle of large meshes than in the middle of small meshes. You can reduce the maximum touch potential while keeping the same number of meshes (and length of wire) by making the meshes smaller around the edge of the station and bigger in the center of the station.

 

[SilverArc]

Hi Jghrist,

I guess, few more moments of your time.

# When fault current returns back to the grid, on what basis we can comment that

" more current flows into the earth from wires that are near the edge of the station than from wires near the center of the station."

&
Touch potential is also higher in the middle of large meshes than in the middle of small meshes.

Thanks again.

Regards,

 

[jghrist]

# When fault current returns back to the grid

Fault current flows into the grid from the fault point (for instance - bus to squirrel to steel to ground stinger to grid). Fault current flows from the grid through the earth to the source of the fault current (substation at remote earth).

I'm not sure exactly why more current flows into the earth from the edges of the station. I like to think of it as the current having more freedom to move out towards remote earth from the edges than crammed into the center, but I'm not sure that this is a valid explanation.

Touch potential is higher in the middle of large meshes because the surface voltage is lower (and touch potentials higher - remember that touch potential equals grid voltage minus surface voltage) when you are further away from the grid wire. The middle of a large mesh is further from the grid wire than the middle of a small mesh.

 

[RalphChristie]

Excellent Jghrist

 

[dgbpeakpwr]

jghrist
"I'm not sure exactly why more current flows into the earth from the edges of the station. I like to think of it as the current having more freedom to move out towards remote earth from the edges than crammed into the center, but I'm not sure that this is a valid explanation."

More current flows where there is a greater difference in potential. Inside the grid area adjacent conductors raise the potential of the ground in the grid area. At the edge of the grid the potential falls off with distance. There is a greater difference in potential and more current flows at the edge of the grid because there is no conductor outside of it also raising the ground potential.

The effect is similar to how ground rods lose effectiveness in reducing grid resistance when placed to close together.

by the way, jghrist
Excellent post

 

[jghrist]

dgbpeakpwr,

That explanation makes more sense than mine. Thanks.

 

[SilverArc]

Hi Jghrist
This time it is not related to grounding,
Just a confusion I want to get rid of created by a peer at work.

If I have a 10 MVA transformer with 7 % impedance at 69/13.8 KV...
The short circuit level at secondary is 5981 A and primary is 1196.2 A. When I know this is the possible primary short circuit in primary as well as seconday, then why do I need the short circuit MVA from Utility.

Could you kindly drop a word...as I am sure I had right but this guy at work screwed my concept.
Thanks for your time.

 

[jghrist]

You need the short circuit MVA from the utility to determine the system impedance ahead of the transformer.

I suggest you first search the forum for "short circuit MVA" using advanced search, exact phrase. If you can't find the answers you need, I suggest that you start a separate thread.