Transferred GPR to underground mine

[MD88]

Hello,

As per CSA M421, and I think several other mining electrical codes around the world, the ground potential rise (GPR) is limited to 100V for underground and open pit mines. Since the GPR at the main utility tie sub is usually much higher than this, the UG mine substation power grounding system needs to be isolated from the plant ground system.

I've previously seen a design where the overhead line from the main plant substation came in to an "isolation transformer", and the ground grid at this transformer was connected to plant ground, but the neutral grounding resistor (NGR) connected to that transformer wye was located a certain distance away from this substation, and connected to a separate ground grid. The shields and ground wire on the phase cables from the transformer secondary to the UG distribution were insulated from ground at the transformer secondary, and connected to the separate grid as well.

I understand how this prevents the GPR from the utility tie sub from being transferred... and faults on the transformer secondary windings to the tank would follow a path back to utility tie sub grid via the skywire, through remote ground, and up through the separate mine ground. The fault current would be limited by NGR of the isolation transformer, and the GPR of the mine would be the Rg of the mine grid X the NGR current of the isolation transformer.

I'm struggling to see why the isolation transformer tank was bonded to plant ground, while the NGR was tied to mine ground (note that both NGRs were the same amperage). If the isolation transformer tank was bonded to mine ground instead, it would be faults on the primary windings to the tank that would go through remote ground and would equally be limited by the utility tie sub NGR (since it was the same amperage), and the GPR of the mine grid would be Rg of the mine grid X the NGR current at the utility sub... I think??

Maybe I'm having a brain fart, or maybe the design I saw wasn't necessary... any input would be appreciated.

 

[Kiribanda]

If you could upload a drawing showing your concerns that will be great.

 

[MD88]

Very good point! Sorry I should have done that right off the bat... I hope this clears up my question, and what my understanding of the situation is. Red is fault path on the isolation sub primary, and blue is the fault path for isolation sub secondary. Thanks very much!

 

[Kiribanda]

I am not sure whether I fully understood your "CONCERN".
In my opinion, both your sketches show the same thing.
I will consider Main sub as the utility sub feeding the mine whereas your "ISO SUB"(I donot know why
you call it as an isolation transformer) is installed at the mine.
Now, the ISO SUB should have its own ground grid designed per IEEE 80 giving safe touch & step potentials.
It is needed because without that ground grid one standing in the area may be subjected to intolerable
step & touch potentials. All exposed (transformer tank etc) & extraneous conductive parts including cable fed
equipment shall be bonded to this ground grid.
The NGR at the wye low side of the transformer is used to keep the Ground Fault Voltage within 100V at the low side.
That being said, to mitigate the transferred potential (GPR) transmitted through cable fed equipment at the low side,
the ISO SUB can have its own ground grid (high side one) mentioned above physically separated (normally 50m) from the low side grid.
In that case the bottom of the NGR should be insulated properly to withstand the calculated GPR.

 

[rzgpp]

Hi,
I think the key is Ingr.plant/Ingr.mine.NGR on plant/mine are the same...but current in second scenario would be Ingr plant,which is higher in subtransient stage,becouse there is no impedance of insulation transformer....even voltages are equivalent.Better scenario is 1!

 

[Chris_Eng123]

Anyone have any good thoughts on how to do this where the utility and mine feed are at the same site (within 100 ft)? The "isolation" transformer is the station transformer, or would a second transformer be required with a ground grid separate from the station grid? With an isolated "mine" ground grid, do we really have to rely on a ground fault path through the earth (as sketched above), for ground faults on transformer secondary, or any secondary switchgear/PCBs in the substation? This sketch below, from the IEEE Green book, implies the same.

 

[lansford]

Lets say the main substation feeds a variety of stationary equipment. If so the Low side grouding of that substation can be solidly, ungrounded or resistance grouding. And you have a neutral run from that substation to the isolation substation or you do not have the neutral run.
The isolation substation has the ground for the resistor located a distance from the substation frame ground which the neutral of the main sustation is connected to the isloation sub frame groound.
Since the high side of the isolation sub is delta connected there is no panth for zero sequence current back to the main sub so regardles of the main substaion configuration, there is that isolation for line to ground faults on the low side of the iso sub.