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Grounding hazard from induced currents under open 15kV lines

Grounding hazard from induced currents under open 15kV lines

Grounding hazard from induced currents under open 15kV lines

(OP)
This is a metals plant that transmits 15kV power from a 100MVA, 138kV/15kV Y-Delta transformer across a gravel area about 300 feet ending in another elaborate switching system.

We've got our guys installing some fans on the outside of metal building that's located 30' from the tower. We had them install some bonding straps to the 15kV tower. We are measuring on that ground wire 2 to 3 volts to ground and a whopping 250A. Our people act like this is no big deal and its safe to work on. Can someone explain why this is so?

RE: Grounding hazard from induced currents under open 15kV lines

A sketch would help us visualize the problem.  What does the metal building 30' from the tower have to do with bonding straps on the tower?  If the straps are bonding the tower to ground, how can there be a voltage to ground?  Where is the 250A? In the bonding strap?  Not likely.  The 15 kV system is ungrounded.  Even a direct connection from a phase conductor to the tower would not produce 250A.

RE: Grounding hazard from induced currents under open 15kV lines

(OP)
Unfortunately I don't have access to a scanner to provide a sketch. The bonding straps run from the siding of the building to the metal tower. The tower is already grounded. As they place additional straps from the building to the tower the current is cut 1/2.

It's happening.  

RE: Grounding hazard from induced currents under open 15kV lines

(OP)
As another possibility let me throw this out. There is also lots of 138kV transmission in the near vicinity. They transmit in grounded wye.

I remember a few years ago I was at a plant that had built a large pipeline under those lines. It also carried a current like this. The utility engineer told us that the transmission lines used the earth as a the return path. I just wonder if this has anything to do with it.

RE: Grounding hazard from induced currents under open 15kV lines

No, they don't use earth as a return path. Transmission lines are balanced three-phase systems and have no "return" current.

We still don't really understand your system so it's hard to say what's going on.

What's in the metal building? You might take a look at it's electrical system and grounding as a starting point.

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Grounding hazard from induced currents under open 15kV lines

(OP)
there are large fans in the building. They don't pick up any abnormal current readings to ground between metal parts and ground inside the building.

So all transmission lines are perfectly balanced 3-phase systems? How is that possible when no load would be perfectlty balanced? I've read one of the definitions of vars as that portion of the generated power not used and returned to the power plant? I truly don't understand...

And those pipelines were literally 1/2 mile from any source of electricity. If you cut into one of them sparks flew.  The transmission lines had nothing to do with them either?

   

RE: Grounding hazard from induced currents under open 15kV lines

Quote (alehman):

No, they don't use earth as a return path. Transmission lines are balanced three-phase systems and have no "return" current.
Oh, don't we wish.  Close, but not exactly, and there is some return current that takes whatever path it can.

RE: Grounding hazard from induced currents under open 15kV lines

bdn,
Think about a single-phase load connected phase-to-phase. Unbalanced line current, but no resultant earth current.

Yes, my statement is oversimplified. The point I was trying to make is that he should not expect to be seeing a large amount of earth current from a transmission line. Obviously there are earth currents due to line asymmetries, geomagnetic affects, etc. but I would not expect those to be on a scale that would induce 250 amps between a building and adjacent 15 kV power pole. Again I still don't fully understand your situation, but if I were going to troubleshoot this, I would probably start by verifying the electrical system in the building is properly grounded.

You'll have to get someone else to explain your pipeline situation. That's beyond my area of knowledge.

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

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