Substation Grounding
Substation Grounding
(OP)
Is this kosher? If you are designing a ground grid and all the equipment in the substation is tapped to two different spots on the grounding grid, can you choose the ground grid conductor to have an ampacity of half the total fault current. For example, you have 30 kA available or are designing to and because the equipment is double tapped to different points of the grid, you can choose the ampacity of the ground conductor to be 15 kA.






RE: Substation Grounding
I wouldn't.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Substation Grounding
RE: Substation Grounding
RE: Substation Grounding
IEEE80 does not recommend the equal division of current in usually 4 paths resulting from the crossing of conductors of mesh.
The difficulty arises from the interaction of magnetic fluxes and their associated electric fields in these intersections of conductors, a situation that prevents the simplistic reasoning to equally divide the current in the mesh of grounding grid.
RE: Substation Grounding
RE: Substation Grounding
And that is not the only problem. "Touch potential" between machines (motors and driven machinery) need not be hundreds of volts to cause major damages to bearings. The damage occurs already at five or ten volts and it is fully developed in the few milliseconds that it takes to trip a breaker or blow a fuse. Seen that quite a few times now.
I am right now looking at those phenomena and it is amazing how bad some ground meshes are after twenty or thirty years operation. Not talking about ground resistance now. That is usually checked regularly. Talking about the integrity of the ground grid itself, and its risers and connections.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Substation Grounding
I did not understand this graphic. It seems to me, there was a shortcircuit at left Iremote. Could you clarify?
Skogsgurra,
Is it an arcing ground?
RE: Substation Grounding
We cannot say. But we definitely know that the usual ground clamps do not work here. The reason is that the time constant (L/R) is much too long for the ground clamps to work properly. So, we use much lower frequencies to induce a few hundred millivolts and then measure the resulting current. We can resolve down to better than 0.1 milliohms using this method. And the whole thing is still portable, using a 12 V motorcycle accumulator.
If arcing or not, we do not know yet. Our problem is that the current distribution at fault seems to be "everywhere". If you have a prospective short circuit current at about 30 kA, which we have. Then, the peak (inclusive the DC transient) can easily be 50 or perhaps 60 kA.
It only takes 1 percent of that current in less than one millisecond to "reliably" destroy, or at least "nick", bearings in motors and if your ground grid (or the risers/downleads) have corroded over the years, you can easily have a lot more than 1 % of the current going the wrong way. It is a little like putting the ground clamp in the wrong place when you are arc welding.
I'm afraid that we are seeing the tip of an ice-berg here. And the observations we have made so far seem to indicate that the problem is a lot more wide-spread than we thought it was.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Substation Grounding
The “main part” of the grounding grid will be calculated for 30 kA but the rest of grid only for 2.5 kA.
RE: Substation Grounding
RE: Substation Grounding