Interconnected Grounding System

[SilverArc]

Hi,

I have a situation, where I might have to consider(As advised by Sr. guys) connecting a new substation ground grid( Grid A) to a near by existing grounding grid(Grid B).

The near by existing grid has no surface installation and is connected to another substation grid( Grid C) located at a distance of around 150 m.

The new substation(Grid A) will be around 10 m from this old grounding grid(Grid B). I have been advised that, connecting these three grids togather will reduce the GPR.
and we might possibly avoid using ground rods in new grid(Grid A).

I tried to surf around for finding a publication on interconnected grounding system. The advise given to me by sr. guys is based on a hearsay and I am sure that connecting all grids togather will neither help reducing GPR nor touch potential in new substation.

It will only help touch potential between new and old grids.

Jghrist,
Could you kindly put a word here. You have a lot of contribution in making me understand this grounding issue.


Regards,

 

[cuky2000]

By definition, GPR voltage is equal to the maximum grid current times the grid resistance. GPR = Ig.Ro.

On the other hand, the equivalent ground grid resistance of interconnected substations is usually less than one substation along, (Req < Ro).

If the increase of the new injected current to ground for interconnected substations is proportionally less than the reduction in resistance, the resulting GPR of interconnected substations will be less than the GPR of the original substation.

Therefore, the advice given by your sr. staff may have valid merits to be considered.

 

[rcw retired EE]

Connecting the grids will most likely provide a lower impedance to remote earth. More copper in contact with the earth usually means less resistance. For the same fault current, the GPR will be less.

 

[jghrist]

You may be increasing the step- and touch-voltages of the existing grids by interconnecting them. Some of the fault current from a ground fault at the new substation will flow back to the source through the existing grids, creating step- and touch-voltages. These may be higher than the step- and touch-potentials for a fault on the existing substation if the fault current is higher.

Conversely, if the fault current at the existing substations is higher, the step- and touch-potentials at the new substation may be higher for a fault at the existing substation.

The interconnected grids can be modelled with more sophisticated software such as that provided by Safe Engineering Services

http://www.sestech.com/

or Advanced Grounding Systems

http://www.ap-concepts.com/win_igs.htm.

 

[JensenDrive]

Side issue; not sure if it applies to your case.

If sub a, b, and c, each had different available fault currents, the grid design in each is based on the the avialable fault current in that sub. However, if you interconnect the ground grids, I think would you now have to design the overall grid in all 3 subs based on the sub with the highest fault current. I do not design ground grids, so this issue is subject to review by other more experienced commentators.

 

[SilverArc]

Thanks All for your comments.

Jghrist,

I am using WinIgs for my simulation.

A conceptual question:

Earth potential profile or surface potential or ground potential, Whatever we call it: In a substation, the physical arrangement consists of 4-6 inch of crushed rock, bakcfill of soil and then grid .5 meter below the earth.

What potential do we see on the crushed rock. Because we walk on crushed rock in a substation or we have roads inside for moving equipment. The crushed rock is an insulator, So will it be correct to say that step and touch potential are of no relevance on crushed rock.

Kindly advise me, if I am correct.

Thanks

 

[rcw retired EE]

Your assumption is incorrect. The step and touch voltages will still be there during a fault whether the surface is crushed rock, pavement or wet native soil. The surface covering only changes the allowable step and touch voltage.

The crushed rock acts as a high resistance in the circuit composed of the substation structure, the electrician's hand, arm, body, foot, shoe, the soil and the buried gid. The resistance of this circuit determines the amount of shock current flowing through the body from a given step or touch potential. By increasing the foot-to-soil resistance, crushed rock reduces the maximum shock current, allowing a higher step or touch voltage.

See IEEE 80 or the IEEE Green book for better explanations.

 

[SilverArc]

Thanks rcwilson.

When you used the term High resistance, that can be implied as an insulator. I agree there will be step and touch voltage but they will loose their significance as there will be no current flow. It is like doing a maintenance on a live high voltage line ?
An lineman wearing an insulator suit.


I did a simulation using a WinIGS, where in an underground mine, a screen mesh was encased in a concrete foundation of a transformer pad, which had a crushed rock below the foundation and then bed rock its self.

Now, if there is a ground fault, software gives me very high values of Grid Resistance, GPR and then Step and touch potential.

But if you analyze this situation from a generalized prospective, the because Current has no way to flow through ground, because the bed rock is an insulator and the concrete as well. The screen mesh is an equipotential surface. The current will flow back through the grounding conductor bonded to the transformer casing even if a human touches it during a fault.

I thought I had grounding completely figured out but as we progress towards new issues questions keep coming up.
Thanks again for your time on earlier post.

 

[cuky2000]

I concurr with rcwilson.

Enclose se an sketch with a graphical effect of the crushed rock over the allowable step and touch voltage.

http://cuky2000.250free.com/Gnd Crush Rock Effect.jpg

 

[jghrist]

SilverArc,

The crushed rock has a high resistance, but is not an insulator. As rcwilson says, it reduces the current going through the body during a fault but does not eliminate it. Similarly, the bedrock will not be an insulator.

The current will flow back through the grounding conductor bonded to the transformer casing even if a human touches it during a fault.

This is true if the secondary winding is the ground source, such as a secondary fault where the secondary winding is solidly grounded wye. Step- and touch-voltages will occur for primary side faults where the return current has to get to a remote source and part of it flows through the earth.

 

[SilverArc]

Thanks Jghrist.

I was discussing this concrete foundation issue with one of the civil engineers and he said that they use a

screen mesh encased in concrete to support foundation and connect it to rebars.

Should I presume that in case of substation grounding, this galvanized steel screen mesh is connected to grounding grid and it does not lead to any hindrance of current flow with in concrete itself?

 

[cuky2000]

Corrosion and concrete crack under fault or lightning strike should be analyzed.

a) CORROSION: Small percentage of the ac current (approximately 0.01%) becomes rectified at the interface of the steel bar and concrete. The threshold potential for dc corrosion is approximately 60 V dc. If properly design, this should not be a concern.

b) FOUNDATION CRACK: High current could evaporate suddenly the moisture in the concrete creating overpressures that could crack the concrete. This risk is present even if the structure is not intentionally grounded since the contact with the steel structure with anchor bolts and the semi conductive characteristic of the concrete may pass high current.

Since appears that you do not have a lot of options, I will work with your civil structural to find solutions to the groundings needs.

 

[jghrist]

You cannot assume that the foundation reinforcing is connected to the ground grid. See IEEE Std 142, Grounding of Industrial and Commercial Power Systems (Green Book) for a discussion of concrete encased electrodes and how to connect the rebar to building steel (which is connected to the grid).

I'm not sure what you mean by a screen mesh connected to rebars. Usually, the foundation reinforcing is rebars that might be arranged in a mesh. Some slabs may have steel wire mesh reinforcing instead of rebars.

I'm also not sure what you mean by the screen mesh not leading to any hindrance of current flow within the concrete itself. If a ground fault occurs to a metal structure that is connected to the ground grid, part of the current will flow from the grid into the screen mesh, through the concrete, and into the soil to the remote ground fault source, if the mesh is bonded to the grid. If the mesh is not bondeded to the grid, then the foundation will act like a big lump of low resistivity soil; some current will still flow through it just like it flows through the soil.

 

[sslobodan]

Well the sole purpose of a grounding system is to take the over voltages to a ground saving your equipment. There are different issues about grounding systems, but most important fact is that in direct grounded system you will have all points of a system on a same voltage level. In case you have large over currents (2 or 3 phase short circuit) that means that all of your gear will be affected and if the over current passes the switching ratio for feeding line (disconnecter will jam due to a to high current and will not disconnect) you will have total disaster in a facility burning everything down. Connected system will only help you spread malfunction. Decreased resistance does not help you in this case to do anything because it will not help you speed up taking out malfunction currents to the ground. Proper calculated system is the only thing that you should do here, and connections should be done separately. If you have problem with high resistivity try to put metal pipes (like for pluming or made for this purpose) length 2.5 - 3m in the ground spread in grounding system. that will decrease resistivity of a system and you will be having only few hundred dollar cost extra for it.

 

[cuky2000]

Hi sslobodan,

I believe the grounding system have multiple objective. One of the main purposes of substation ground grid is for personnel safety.

In general the objective of a grounding system may be summarized as follows:

1. To provide safety to personnel during normal and fault conditions by limiting step and touch potential.
2. To assure correct operation of electrical/electronic devices.
3. To prevent damage to electrical/electronic apparatus.
4. To dissipate lightning strokes.
5. To stabilize voltage during transient conditions and therefore to minimize the probability of flashover during the transients.
6. To divert stray RF energy from sensitive audio, video, control, and computer equipment.