Grounding a large oil filled transformer in a graveled diked area.
Grounding a large oil filled transformer in a graveled diked area.
(OP)
We are installing a large new oil filled transformer and a piece of equipment it feeds in a spare bay at a large industrial facility. It will sit in the middle of a graveled area that is a containment dike for every oil filled transformer that is in the row. There are several. I've tried to show it on the attached sketch.
The equipment manufacturer is calling for a grid type switchyard grounding method to form an equipotential plane and a ground resistance of less than 1 ohm. I'm confused on how to do this since there is no way we can penetrate the dike or get to either side of it. And forming a grid under the gravel seems pointless, as it isn't in full contact with the ground.
What we are thinking to do is tie all the rebar in the new foundation together and go to the existing ground cables. I'm posting this just to see if any one has any suggestions. Thanks.
The equipment manufacturer is calling for a grid type switchyard grounding method to form an equipotential plane and a ground resistance of less than 1 ohm. I'm confused on how to do this since there is no way we can penetrate the dike or get to either side of it. And forming a grid under the gravel seems pointless, as it isn't in full contact with the ground.
What we are thinking to do is tie all the rebar in the new foundation together and go to the existing ground cables. I'm posting this just to see if any one has any suggestions. Thanks.






RE: Grounding a large oil filled transformer in a graveled diked area.
check against NFPA-30 if the distance to the Building and to "road" are legally.
Since you cannot extend the Grounding Grid beyond the road and the building can you extend it left and right hand?. You have to check
the touch and the step potential as per IEEE-80/2000 so you need to know the short-circuit current through the Ground.
Depending on earth resistivity you could also reduce the total resistance. The new foundation rebar will not help you to lower the Ground Resistance.
RE: Grounding a large oil filled transformer in a graveled diked area.
RE: Grounding a large oil filled transformer in a graveled diked area.
It doesn't sound like we can do anything other than do some major work: cutting into paved areas or under the dike if we really have to get the resistance down to the required 0.1 ohms. I'll find out from the manufacturer what's the issue.
Another issue that has come up on this is that there is actually current flowing on this ground grid at all times. They say it is due to the induced voltages from all the open bus stuff in the vicinity.
Grounding the rebar will help create the equipotential plane though correct?
RE: Grounding a large oil filled transformer in a graveled diked area.
RE: Grounding a large oil filled transformer in a graveled diked area.
RE: Grounding a large oil filled transformer in a graveled diked area.
RE: Grounding a large oil filled transformer in a graveled diked area.
Specifically after checking the drawings...the bottom phase is about 30" from the concrete. The transformer and rectifier set mfg are the same. Per the nameplate on the transformer the secondary amperage is about 25kA. The only instructions the mfg has given us is to put an isolation pad (1/2" of specific material) under the transformer and under the rectifier set. I'm showing this on sketch I attached in the original post. Do you still think this would be an issue? I haven't heard of fiberglass rebar, is it strong enough and do think this would still be preferred?
RE: Grounding a large oil filled transformer in a graveled diked area.
Fiberglass rebar is available - not as strong as steel, so it would have a larger cross-section.
RE: Grounding a large oil filled transformer in a graveled diked area.
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If we learn from our mistakes I'm getting a great education!
RE: Grounding a large oil filled transformer in a graveled diked area.
The transition between the transformer and the rectifier is surrounded with a mfg supplied silicon sleeve. The connections are copper plates coming out of the transformer that directly match to copper plates on the rectifier, where they bolt together. I don't consider that isophase, at least with what I'm familiar with as iso-phase.
There are actually 2 transformers in one case, one delta-wye the other delta-delta to create the AC for a 12-pulse rectifier, so there are 6 connection points. The mfg has said nothing about the iron concern and they have seen and know exactly where this is going. I appreciate the responses though.
RE: Grounding a large oil filled transformer in a graveled diked area.
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Grounding a large oil filled transformer in a graveled diked area.
RE: Grounding a large oil filled transformer in a graveled diked area.
The isolation material is a trademarked product called Inbord-E. It's described as a paper laminate with melamine surface.