Fence grouding at a generating facility

[TurbineGen]

I have recently been given the task to design and construct a grounding system for a metal chain link fence at a generating facility. I have never done this before so I'm looking for some help. The fence is 300 feet long and is more or less a straight line. What value should I be targeting for a fair ground resistance? I plan on connecting to the station grounding system. Could that be a bad idea?

The voltage of the generators is 13.2 kv, and is then stepped up to 230KV before it leaves the staion. The power generated is about 400 Mega Watts. Our generators are high impedance grounded and the transformers are soildly grounded.

Thanks in advance,

-TurbineGen




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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[GTstartup]

You would have to complete a step and touch potential calculation to determine this.

On a lighter note, I had a similar assignment and had a huge fight with the contractor when I asked him to remove the vinyl coated fence he installed and replace it with a non coated type.

 

[jghrist]

Some utilities ground the fence to the grid, some isolate portions of it because it often runs quite a distance from any ground grid. If you ground it, you will probably have to run a perimeter ground wire on both sides to limit touch-potentials unless one side is next to the station ground grid. See IEEE-80 for pros and cons on grounding or isolating fences. As GTstartup says, you have to do the step- and touch-potential calculations.

 

[rcw retired EE]

Connecting to the plant grid may be a bad idea if the fence is outside of the grid. The metallic fence will carry the Ground Potential Rise (GPR) of the plant grid outside the grid area, possibly resulting in hazardous tough potentials during a fault.

Breaking the fence continuity by installing an isolating section at the edge of the main grid might help. But ground cables on both sides of the fence are probably needed, out about a meter and 0.5 - 1 meter deep. Anyone touching the fence should have a ground grid under their feet.

A simulation analysis is needed to get an accurate picture of the hazard. Read the applicable sections of IEEE 80 for advice, but it is sketchy.

 

[SilverArc]

GTstartup,

What is wrong about Vinyl coated fence ?

Thanks

 

[rcw retired EE]

Nothing wrong with the vinyl coating as long as it is removed at the locations where the fence fabric is connected to earth or ground leads. The vinyl does stink when you Cadweld it.

 

[JensenDrive]

For chain link fencing, if you vinyl coat the wires, the vertical wires may be insulated from one another. That does not seem too desirable.

 

[GTstartup]

Exactly Jensen.

Basically if you procede as described in RCwilson's post only the wires that are scraped are grounded.

 

[SilverArc]

Jensen,

"the vertical wires may be insulated from one another"


So how does this matter. I am working on a substation project and fencing might be coming shotly in my way.

Is there are any literature available to get a better feel for it.

Thanks

 

[TurbineGen]

All of you, thanks for the advice. I will do the step and touch potential calculations

The fence runs away from the power station 500 feet or so. The plans I made call for a non conductive fence after 300 feet as there are overhead lines that cross this fence 125 feet away from the plant (giving an extra 175 feet of grounded fence beyond the lines). The grounding cable I'm using is a 500kcmil cable connecting the ground rods only on one side of the fence (the other side is a rocky cliff). About 50 feet of the fence runs alongside the station ground and then goes outward from there.

Should I isolate the fence that runs beyond the grid by installing a non conductive fence between the metal fence beside the station and the metal fence that runs outward? Also, it appears I can only run a ground cable and rods on one side of the fence as the other side is a rocky cliff. If I do isolate part of the fence, how do I lower it's resistance when I can only run cable on one side? This fence encapsulates a parking lot which (and I have yet to verify this) has a 500kcmil loop around it on three sides (the missing side being the fence side). If that loop is in fact there and connected to the station ground, should I connect to it? Or should I try to isolate the fence run entitrely? I tried my best to make a "text picture" below to give a description of what I have.

============================================
fence |
(station)| |
| |
______| |
| |
| Existing cable |
|_____________|

Again, thank you all for your support.

-TurbineGen

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Zogzog]

IEEE 142 says generating facillities should be 1 ohm or less.

 

[TheBlacksmith]

If the wires are insulated from each other and not grounded, voltage may be induced in them and if the vinyl is broken where someone touches it, they could be shocked. The Navy proved this a long time ago with metallic life lines and ladders that weren't properly grounded and in the vicinity of high power sources.

 

[GTstartup]

One of the reason for grounding a substation fence is so that in the event of a fault the fence and the ground at that location remain at the same potential. It is not practical to connect a ground to every fence wire or post so it is permissable to ground selective wires and posts and allow the fence's natural conductivity to take care of those components not directly connected to ground. If the fence is made of insulated wire, then it is not electically continous. In this case you are relying on the wires insulation to protect personnel. Generally it's not rated to do that.

 

[rcw retired EE]

Turbine Gen- Based on your sketch I recommend grounding the fence and connecting the ground to the existing ground wires around the parking lot. Not connecting it could create more of a hazard. Where the fence leaves the parking lot and substation area, an insulating section may prevent transferring the grid GPR outside the grid area.

Since one side of the fence is a cliff, it may not be practical to put a buried conductor on that side. You would rely on the ground conductor on the other side to keep the fence near the local ground potential. We ground the fences to prevent someone from getting shocked during a HV ground fault when their feet on the ground and their hands on the fence are at different potentials due to the current flow in the ground. With a cliff on one side, the odds of someone standing there during a fault are very small. I'm guessing that the current flow in the ground at the top of the cliff would also be reduced. (Where is it going to flow to?)

There are other factors to consider – what is the GPR of the grid, what is the maximum allowable touch potential given your fault clearing time and type of soil at the fence? Evaluation of all the criteria will help you evaluate the risk and mitigate it where necessary.

GT Startup- Good point about isolated fence fabric. I'm wondering if the vinyl coating also reduces the touch potential hazard. The uncoated posts and horizontal pipes are connected to the buried rod or ground wire but the fence fabric would be floating.

This is a problem near the HV lines where the fabric can get potentials induced. But I don't see it as a problem on the plant perimeter fence that is 50-100 meters away from any line. The insulated fabric would not transfer the GPR so it would not create a touch potential hazard and there is nothing in the area to energize the fabric.

 

[jghrist]

If the existing cable is connected to the station grid, then it does no good to isolate the portion of the fence that runs next to it or is bonded to it. The station grid potential during a fault will extend out to the fence through the existing cable.

If the existing cable is not connected to the station grid, it may still have a high voltage induced in it. I suspect that you will be better off connecting the existing cable to the grid and bonding it the the fence. If one of the lines falls on an isolated fence, there will be a larger danger.

If the 175 feet of fence beyond the parking lot is truly non-conductive, like a wood fence, then I wouldn't worry about it. If it is vinyl coated steel, I wouldn't consider it non-conductive.

On the insulted fabric question, if the coated wires are next to grounded wires or surrounded by a grounded frame, I'd treat them as if they were the same potential as the frame. What other voltage would they be? The vinyl coating will not reduce the touch-potential hazard, but in practical terms, I don't think it will make it worse.

 

[GTstartup]

From a ruling by the NESC on rule 092E - Point of Connection Grounding Conductor, Fences.

“The intent of Rule 092E is to provide an intentional, conductive path to earth for all metallic fence components,
either by direct connection to a grounding conductor or by bonding to another suitable grounded fence
component. Note the definition of bonding: “The electrical interconnection of conductive parts...”

 

[GTstartup]

Sorry,

Here's the link

http://standards.ieee.org/nesc/NESCIR528.pdf

 

[groundperfect]

I personally would drive deep earth elecrodes to attain a resistance near 1 Ohm or less to better protect the whole facility incluing your fence. Not knowing the exact layout of the area I cannot say how many or where to place them and connect them to the equipment. Before everyone laughs as you said this is in a rocky area, we have been installing rods over 100' deep in solid rock for many years with special equipment. Deep earth rods, especially in a rocky environments have many advantages. Being in a rocky area a deep earth ground will be more consistant year round where shallow systems will be dependant on ever changing conditions. We have been to several locations where shallow grounding systems never reach a desiable resistance no matter how far they sting it out or how many 10' rods they hammer in the ground. Spacing deep earth rods out nearly as far as they are deep will give much better coverage than any grid mat, ring, or other type or ground system. Check out

http://www.groundperfect.com