Grounding a substation located on a bedrock 1

[so0774]

Hi,
I'm designing grounding system of a 25KV substation. However, I have been told that the whole plant is located on a bedrock(at least minimum 13m deep). Driving 20ft ground rods will not contribute much to my grounding design.
What other options do I have in order to improve my grounding?

Thanks

 

[cuky2000]

OPTIONS:

- Ground the concrete foundation and rebars.
- Using grounding plates.
- Ground any additional supplementary burried metallic structure (pipes, tank, etc)
- Enrich the soil with bituminus materials or commercially available conductive soil with moisture reteention.
- Use conterpoises.
- Consider a ground electrode.

Last resources:
- Chemical electrode environmentally acceptable.
- Provide continue mositure source.
- Limit the short circuit and speed of protective devices.

 

[so0774]

Thanks cuky2000,
The problem is the path to earth. As I mentioned the whole plant is on a bedrock (which acts more like an insulator), so even if I use ground plates or ground electrodes in significant numbers, I won't have much contributions to my grounding.

 

[cuky2000]

The switchyard and plant grounding should be interconnected. The plant slab and deeper foundations will help significantly if will consider as part of the overall ground grid.

One of the problems that you may face is to prove how much this help. There are commercial software with capability to model the foundation and concrete electrodes. However, those software are not inexpensive.

See if the enclose link could help.

http://cuky2000.250free.com/GND_Electrode1.JPG?view_image=1

 

[JensenDrive]

Maybe a ground grid expert can discuss the theory a bit. I am not sure abut the point of a low ground resistance. Is the point to create low step and touch potential? If you make your ground grid uniform, and any two surfaces that a person can touch will have a low potential difference during a fault conditions, then is that what you are after? In that case, it does not matter if the plant is on a perfect insulator, as long as the top couple of feet a person can touch are relatively conductive. What is the point of driving deep ground rods in this perspective?

 

[jghrist]

JensenDrive has the right idea. The main purpose of the substation ground grid is to create a relatively uniform surface potential to limit step- and touch-voltages during a 25 kV ground fault.

Good interconnection with the plant ground will provide a low impedance path for plant ground faults to return to the transformer source.

Ground resistance can be important for lightning protection and to limit GPR (Ground Potential Rise) if there are remotely grounded communication lines coming into the plant. Isolation of the communication lines may be a more economic solution to GPR problems than reducing the ground resistance.

I doubt that you can economically reduce the ground resistance below what you will get with grounding concrete foundations (Ufer grounds), a substation grid sufficient to limit step- and touch-voltages, and interconnection with the plant grid (probably governed mostly be a ground ring).

The problem might be if you have some arbitrary low resistance requirement imposed by specification or code.

 

[jnunner]

I have a concern with your statement: "I have been told that the whole plant is located on a bedrock(at least minimum 13m deep)."

I am assuming that soil resitivity measurements weren't taken at this site.

A good grounding design starts with soil resivity data, gathered from testing done on site.

For example; while bedrock usually has a large value of resistivity, there may be a water table or some other feature of the soil or the bedrock which could improve your grounding situation considerably. I once performed testing on top of bedrock which turned out to be quite porous, and as a result, appeared to have the same resisitivy as the water table above it.

As cuky2000 pointed out, there is commercial software (we use CDEGS which is quite pricey) which could be used to aid in the analysis of the design. Some of the cheaper software will yield decent results but is usually limited to a maximum of 2 soil layers.

 

[so0774]

Thanks for your comments.
I'm using ETAP ground grid package for my simulation which is not as powerful as CDEGS for Grounding applications. It can only handle 2-3 horizontal soil layers.

I do have soil resistivity results. The problem is that the site is located in Northern Canada. Typically at these locations you will have your top soil layer frozen (sometimes up to 7-8 ft). The bedrock is at least 35 ft deep and it starts from 1.5 ft below the surface. So the water table is at least at 40 ft depth from the surface if not more.

One option that was suggested to me was to find a location at a reasonable distance from the plant in order to place my ground grid and ground electrodes and then tie the plants grounding system to that grid.

 

[jghrist]

A separate ground grid and ground electrodes may improve step- and touch-voltages at the 25 kV substation but if a large part of the return current flows through the separate ground grid, you may have step- and touch-voltage problems there.

There may be voltage drop during faults along the conductors connecting the grids. Most grounding programs will assume an equipotential ground grid, so modelling the two grids with interconnecting wires may be inaccurate. CDEGS uses a separate (more expensive) module MALTZ to evaluate grids where the potential may be different at different points on the grid conductor.

If the main reason for the separate grid is to lower the ground resistance, you have to evaluate the reason for needing the lower resistance. The separate grid will be of little use for lightning protection because the connecting wires will be a very high impedance to high frequency lightning surge currents.