Grounding of Skid Mounted Substaions

[Kiribanda]

Hi,
In an outdoor air insulated substation where an overhead transmission line is terminated,
the underground ground grid (mesh)consists of bare copper conductors and rods. This design
ensures that during a ground fault when the ground fault current is flowing back to the
remote neutral through the soil, a person standing on the soil (gravel layer)
is not getting touch & step potentials exceeding the tolerable values including the
GPR & transferred potential.

I have seen photos of several skid mounted substations with an HV line (mostly 72kV)
terminated to these outdoor skids.These skids are having hot dipped galvanized gratings as their
surface layer on which a person is standing.Since the person is standing on
an equipotential surface, how does the designer control the GPR to be within limits? Is
GPR issue a concern for these skids? How does the ground fault return current is passed
to the remote neutral?

Thanks

 

[waross]

Those are equi-potential mats, typically placed where a worker will stand to operate an overhead switch. They are NOT grounding mats. The ensure that a person standing on the mat to operate a switch will be at the same potential as the switch handle.
They have only one connection, to the switch operating mechanism.
Some utilities carry equi-potential mats or grids on the service trucks.
These are deployed and connected whenever a pole top disconnect is to be operated.
They may be on the surface or may be covered with a few inches of crushed rock.

How does the ground fault return current is passed
to the remote neutral?

It doesn't. That's the point. If the mat was connected to the ground grid so that fault current flowed to and through the mat, there would be an IR voltage drop in the connecting cable and the touch potential between the switch handle and the mat may rise to lethal levels.
Anecdotally, I once saw a new installation where there was a jumper from the mat to the main ground grid.
I voiced my concern and the next day the jumper was removed.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

If the mat was connected to the ground grid so that fault current flowed to and through the mat, there would be an IR voltage drop in the connecting cable and the touch potential between the switch handle and the mat may rise to lethal levels.

If the ground mat is connected to the handle with #2/0 copper and the mat is connected to the ground grid, in the worst case (ignoring parallel paths), the IR drop would be only 24 volts with 50 kA of fault current. This is based on 6 ft total length from handle to grid, and 0.08 ohms/1000 ft resistance.

 

[Kiribanda]

Jghrist,
Does it mean that for this kind of skid mounted subs too there is a ground grid designed per IEEE-STD-80
under the skid. Is it still required? Since a person is standing on gratings which is an equipotential
surface (no touch & step potential issues), can't we put four copper ground rods at the four corners of the
skid and bond the skid to them?

 

[waross]

Respectfully, jgrist, other numbers will give other values of IR drop.
The lengths are more like 6 to 8 feet from the handle to the mat and more from the mat to the grid.
The cable from the switch handle is not intended to carry fault current and is often much less than #2/0.
Another possible issue is poor connections, particularly buried connections between the mat and the main ground grid.
Also, the handle and operating mechanism are not designed as current carrying objects.
In the event of fault currents flowing down the operating rod to the handle, there is a possibility of arcing at a mechanical joint in the linkage.
An equi-potential mat is never allowed to become a part of the grounding circuit.
Then the current is negligible and possible issues are avoided.
Why waste money and labour to install a conductor which gives rise to the possibility of problems which are not present in a proper installation?
Kirbanda: The equi-potential mat has nothing to do with grounding the sub.
The equi-potential mat only protects a person standing on it. It does not protect someone touching a part of the sub frame or skid away from the eqi-potential mat.
Some equi-potential mats are permanent, some are not.
Some equi-potential mats are carried on the service truck and laid out and connected whenever a pole top disconnect switch is to be operated, at a sub station or in the field.
The mat does not ground the sub and is not part of the grounding system.
The principle of operation is somewhat similar to the principle of a Faraday cage. (But not exactly)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

Does it mean that for this kind of skid mounted subs too there is a ground grid designed per IEEE-STD-80 under the skid. Is it still required? Since a person is standing on gratings which is an equipotential
surface (no touch & step potential issues), can't we put four copper ground rods at the four corners of the
skid and bond the skid to them?

You still need a ground grid meeting IEEE std 80. The switch mat only protects someone standing on it operating the switch. Someone standing next to the skid mounted substation will not be protected if there is a high side fault to ground. He will be subject to step-voltage and touch-voltage (if he is touching the equipment).

 

[waross]

You still need a ground grid meeting IEEE std 80. The switch mat only protects someone standing on it operating the switch. Someone standing next to the skid mounted substation will not be protected if there is a high side fault to ground. He will be subject to step-voltage and touch-voltage (if he is touching the equipment).

I agree with this completely jgrhist.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

Below is a typical ground (earthing) ring suggested for skid mounted substation application primarily to control the touch potential.

 

[Kiribanda]

Hi Cuky,
Thanks. With the gratings, touch & step potentials are not issues. How about controlling the GPR & Transferred potential
during a ground fault on the skid? How can one keep it lower with a set of rods as shown without having a copper ground
grid under the skid?

 

[cuky2000]

Kiribanda,

The issue is not standing on the metallic platform that might be considered as an equipotential surface.

Standing on the native soil and touching the transformer or other metallic parts there is a potential safety concern that needs to be addressed understanding the native soil resistivity for each location, the max. ground fault current and the clearing time of the remote protective device expected to be high for this type of installation.

If there is no soil resistivity data and calculation available for each location, will be prudent to consider a ring loop or other mean to control the touch potential. This ring can be moved to each location and even using cheap grounding material such as metal clad, steel conductor since short term corrosion may not be a significant issue.