Generating Station Grounding - Isolating Building as Safety Measure? 1

[WindTurbinesAreFine]

Hello,

I have been reading this forum for quite some time, and have just recently signed up as a member.

First, a little bit of background on the issue:

We are doing a grounding study for a large generating station. This generating station has recently been upgraded with the addition of a new natural gas fired station adjacent to the old coal fired station and switchyard. The old coal fired powerhouse was shut down a number of years ago, and all of the equipment has since been removed. The switchyard is located directly behind this old powerhouse.

The city is trying to remediate this particular area from an industrial area into something 'green' and usable by the general public. The local utility has sold the powerhouse building to another company that wishes to turn the building into a recreation facility, or something else for use by the general public.

We are providing this study to the local utility as an independent third party study. We have modeled the grounding system as per the drawings provided, and input the test data that we had commissioned.

Here is the issue:

The study that another party has submitted to the utility has suggested that severing all grounding ties from the powerhouse to the main station grid would be the most effective way of ensuring safe touch and step potentials in the powerhouse building. We do not agree with this philosophy, and are actually recommending that the utility adds additional grounding around the perimeter of the building, and ensure that everything is securely bonded to the grid.

My question is:

Has anyone here had any experience in effectively isolating a building or structure that is very close (4 or 5 meters) to a main station grid? Is this even a practical solution? I'm concerned that during a fault situation, there would be enough magnetic coupling between the main station grid and the isolated powerhouse to induce an unsafe touch potential.

I am much more comfortable recommending a solution that is backed by the IEEE (solidly bonding everything) than to go with a solution that seems to be unsupported.

The fact that the general public (as opposed to trained utility workers wearing typical PPE) will regularly be on site, safety should be the main concern.

 

[WindTurbinesAreFine]

I would also like to mention that we have carried out this study in accordance with IEEE Std. 80-2000 as well as IEEE Std. 665-1995. Our recommendations are based on these standards.

 

[ScottyUK]

A drawing would be helpful if can upload one.

Initial thoughts are that if you can't keep the GPR and step / touch potentials within acceptable limits in the publicly accessible area then you need to extend the earth grid until they are acceptable. I guess the remediated area is unlikely to be covered in crushed rock either, so you have a further problem.


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If we learn from our mistakes I'm getting a great education!

 

[xxjohnh]

It so close it can not be isolated. IEE665 states you don't have to bond grids if the separation is more than 200M.
But if you consider if it was a green field site you would not do anything beyond the power plant property line, which would be a grounded fence..

 

[WindTurbinesAreFine]

Thanks for the responses thus far.

Please see attached picture for reference depicting the existing conditions.

As you can see from the picture, the switchyard is in very close proximity to the building that is proposed for public access.

I am of the philosophy that solidly bonding this building to the main station grid would be the safest, and most conservative measure.

Further to these other issues, due to the age of the building, the utility was not able to provide any drawings depicting the existing grounding layout around the powerhouse building, or adjacent yard. This means that I have no idea what grounding conductors (if any) exist in these areas. Site testing revealed that the buildings' ground bus was in fact already tied to the main station grid. It also showed that the outer perimeter fence was isolated from the main station grid. I had to model the powerhouse building as a large concrete electrode, having a resistivity of 30 ohm-m.

We are recommending that they bond everything. The building, the fence, metallic pipes, everything. This may cause the earth potential in some of these 'remote' areas to rise, but at least they all rise to essentially the same level so there is no dangerous difference in potential (bird on a wire philosophy).

The other party that is recommending that the building to be isolated claims that they have physical methods for such isolation.

Aside from picking the building up, and inserting some magical barrier all around the building foundation, I don't see how they can expect to achieve true isolation.

I am not trying to say who is right, and who is wrong. If there is an effective method for isolating this building, I would love to hear it. This forum is filled with professionals with vast experience and knowledge, and as a junior engineer, I have much to learn (and am very eager to do so).

 

[VTer]

This is a little of base but it can maybe put some insight to your situation. We recently also had trouble with manufacturer providing 500kV impulse generator that we are installing state that they want a completely isolated grounding system for this equipment to avoid sending surges throughout the building power and control systems upon equipment failure. Since this equipment is going into the HV test lab inside existing building, we really had a hard time trying to figure out how to “truly” isolate this grounding from the building. Anyway, since this equipment requires a grounding resistance of 2 ohms we decided to install enhanced grounding rods 30 feet deep inside the HV room dedicated just for this equipment. To isolate ground from the building , the first 10 feet of this ground rod was enclosed in a PVC pipe to avoid picking up any surface charges from the building and also directly isolating the first 10 feet from the surrounding soil. We figured that after first 10’ assuming we cleared any foundations the building will be out of the influence zone of the ground rods and isolated. All other grounding equipment inside the room was isolated using glastic or similar stand-off insulators and supports.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[xxjohnh]

You could propose some expensive step and touch potential measurements. How can you install anything on property you don't own. Paving the area would solve it till it got dug up.

 

[WindTurbinesAreFine]

This generating station is right on a lake, so I am a bit concerned that the water table being so close to the surface would effect the influence zone, acting like a direct connection to the isolated facility. Also, the soil surrounding this powerhouse is heavily contaminated by coal dust.

Perhaps my understanding is a bit off, but I'm just not sure how you can be truly considered 'isolated' until you are far enough away where the GPR falls off completely at 'remote earth'. Wouldn't the influence zone extend just as far downward as it does horizontally (like a sphere)?

We certainly could propose more testing, but I'm not sure how keen they are to spend the extra money. I agree, it would give an excellent indication of the touch and step potentials to be expected.

I should clarify, the building has not been 'sold' to another party, but rather it has been leased, so the utility would still be responsible for installation of any additional grounding.

I suspect that a large part of the yard will be paved for parking, but the reminaing yard will likely be nicely landscaped so it is more appealing.

 

[jghrist]

The earth potential during a fault will fall off quickly with distance from the grid. A full analysis using something like SES CDEGS software would be required to determine what the potential of the powerhouse would be. To isolate the powerhouse grounding from the grid would require removing all connecting wires, not just disconnecting them.

The problem with bonding the grounds is the possibility of dangerous voltages introduced by outside wires like telephone in the powerhouse. The telephone wires would be at remote ground potential and the switchyard, if it is connected to a remote source (like a transmission line) could have a high GPR with respect to remote ground.

The new gas fired generator source will probably not cause much GPR because practically all ground return current will flow through the metallic ground connection instead of through earth.

 

[WindTurbinesAreFine]

I would love to have access to CDEGS! We have been using WinIGS in the office for a few months. It is a nice upgrade from SKM GroundMat, but almost seems to lack a finishing quality to it.

Even where the software seems to indicate that isolating the building is theoretically a safe method, I just don't know how practical it is.

It certainly is true that grounding is an art. There are many different ways of achieving the same result.

 

[marks1080]

Based on what you describe, I would venture to guess that removing the ground grid in the power house and isolating from the gas plant would be fine. Numbers would need to be crunched obviously. If your simulations say that isolation would be good enough why do you want to bond the grids together? Isolating transformers aren't cheap.

 

[marks1080]

Opps... i meant to say neutralizing transformers.

 

[WindTurbinesAreFine]

I guess I am just hesitant to cut this building away from the main station grid...it seems counter-intuitive to me.

From a public safety (and liability) standpoint, it just seems safer and more conservative to remain tied to the main station grid. Where there is such a close proximity between the switchyard and powerhouse, it would keep the potential gradients flatter to remain tied. If the powerhouse is isolated and there is an inadvertent connection between the two, the potential difference could be quite high.

Bonding everything may in fact be a costlier solution, but it seems to be the safest.

 

[marks1080]

It might in fact be safer to isolate. You wont know for sure without a proper study.

 

[jghrist]

Another consideration is how the powerhouse gets its electric power. If it is served by a distribution line out of the station, then the distribution line neutral or cable shield will connect the building ground to the station ground grid anyway.

If this is the case, isolation may not be practical. You may have a transferred potential problem, but it's one that could exist at any customer's premises near the station. And one that is usually ignored for lack of any practical way to deal with it.

How would bonding the grounds improve safety? If the grounds were truly isolated, then I suppose there could be a touch voltage problem for someone outside the building that touches the building. This would exist even with the grounds bonded unless you put in a perimeter ground wire. The perimeter ground wire would fix this problem with or without bonding the two grounds.