Remote Substation Ground

[SilverArc]

Hi,

I was a little hesitant in asking this question because I guess this aspect of grounding has been discussed many times.
It is in relation to a mining application, where a 69/4.16 KV substation is feeding a indoor switchgear located in a Hoist-room, which has other electrical equipment also. This substation is 1.2 KM away from the switchgear.

As being an indoor switchgear of 4.16 KV, I guess I should not be much concerned about the GPR as it would never exceed 2400 V only and only if the ground from the substation is not brought to this switchgear.

If I connect this ground to my local grid, then I need to consider the potential my grid will be subjected to because of 69 KV ground fault. I will be considering 3 phase fault current for the calculation instead of single line to ground just to get a better result

What would you suggest, that do I have to or I should connect the two grids togather.
Thanks

 

[SilverArc]

Hey..Everybody

I guess some of you might have some comments.

Thanks

 

[jghrist]

Usually, the grids will be connected by 4.16 kV neutrals, cable shields, or grounding conductors that are dictated by other criteria such as ground fault protection. If there is no ground connection, then you may not get sufficient ground fault current to operate protection. Consider if you have a 5 ohm station grid resistance. As you say, you will not have more than 2400 V GPR, but then the fault current would be limited to 2400/5 = 480A by the grid resistance.

The amount of potential transferred to the remote 4.16 kV station for a 69 kV fault depends on many factors such as the grid resistance of each station, soil resistivity, size of the ground connection, number of grounding points of the ground connection, and the resistance of the grounding points.

I don't see how using 3 phase fault current in the calculations will give better results. You are concerned with ground current does not occur in a balanced three phase fault.

 

[waross]

I would hope that your exposed metal equipment at the switchgear is solidly connected to your local ground grid.
If you connect the switch gear ground grid to the sub ground grid, the point of connection at the switch gear is important.
You should connect grid to grid. In this way, even though a 69Kv may raise the voltage of the ground grid at the switch gear, the switch gear ground grid and the exposed metal of the switchgear will remain at the same potential.
If the ground tie is connected to the frame of the switch gear, then there may be current flow from the frame of the switch gear to the local ground grid in the event of a 69KV fault. This current will develop a potential difference between the switch gear frame and local ground. In the event of a high resistance developing in the conducting path from the switch gear frame and the local ground grid, a fault on the 69KV system may develop lethal potential differences between the switch gear frame and the local ground.
respectfully

 

[SilverArc]

Thanks Jghrist and Waross.

Here is a graphical description of the situation

http://img107.imageshack.us/my.php?image=drawing1my6.jpg

If I dont bring this neutral to my 5 KV switchgear, what kind of problems I can run in to. I have already created a local ground.
I can use the ground of 5KV switchgear to detect a ground fault at the switchgear and get the protection running.

My issue was to find out the pros and cons of not connecting two grids togather.

Jghrist and Waross thanks again.

 

[Alex68]

Usually connecting the two grids is better to avoid transferred potential.
I don't know if 69kV system is grounded and how. I don't know many things of your system, but let's analysing the following case:
Suppose you have a 1ph fault at the main subst. This causes a big GPR since the ground grid resistance is high.
The potential rise is transferrd to the 5kV SWG by the neutral which is not connected to the local ground.
Is there a dangerous situation? Maybe
If somebody is touching the neutral wire while its feet are at the local potential.
That's why I prefer connecting together the two grounding grids.

 

[jghrist]

You have to either ground the neutral at the 5 kV switchgear or run a separate grounding conductor to tie the two grids together. Otherwise, you have no return path between for a ground fault at the switchgear except through the earth. This will most likely prevent proper ground fault protection at the main substation.

 

[SilverArc]

Thanks Jghrist.

I guess you just summed up the whole problem.

Just another point, In HV transmission lines, where there is no neutral only a skywire, whose resistance eventually increases because of long runs that current distribution is more from earth then skywire.

What is done to operate a ground fault protection between two substations where there grids are connected only through skywires.

I am just trying to understand how this logic applies there.

Regards,

 

[jghrist]

The earth return path for HV is not as restricting as for MV. As a very simplified example, at 115 kV, 66,400 volts to ground divided by 5 ohm resistance is 13,280A, but 2,400 volts to ground divided by 5 ohm resistance is only 480A.