Grounding a large oil filled transformer in a graveled diked area.

[bdn2004]

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.

 

[7anoter4]

As resulted from your sketch the new transformer is not so "large" [something like 1000 gal tank].Nevertheless one has to
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.

 

[bacon4life]

Is this just the default instructions from the manufacturer, or specific directions for this piece of equipment? I have not seen a transformer manufacturer specify the details of the grounding method. Before attempting to meet the fairly onerous requirement of 1 ohm, I would definitely ask the manufacturer for their justification of the requirements.

 

[bdn2004]

The transformer is 30MVA and holds about 5000 gallons of oil. This is a special transformer made for a DC rectifier set that it feeds. The distances to the road and building must surely be ok because there are many transformers of this size or bigger in the same lineup performing the same function. And it can't go left or right as that is the same 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?

 

[dpc]

I'm used to seeing a "no-iron" zone around the secondary side (high current) of rectifier transformers. The high current ac can induce damaging current in rebar. This is for the ac portion. The high current dc is not as much of a concern - unless the rebar is moving, I guess.

 

[bdn2004]

It's not like secondary of the transformer is going through a concrete encasement. The secondary is directly coupled to the rectifier set with the bottom phase 18" or so off of the top foundation slab. Do you see that as being a problem...?

 

[dpc]

It's the short run between the transformer secondary bushings and the rectifier that was the concern. My experience was with larger units and very high currents (40,000 A). The ac bus was several feet above the slab, but is was still a concern. Fiberglass rebar was used. Just bringing it up as a heads up. You need to go by the transformer manufacturer's requirements for a special application such as this.

 

[bdn2004]

dpc,
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?

 

[dpc]

If the transformer supplier doesn't address it, I wouldn't worry about it. You could ask them, just to be sure. I recall the drawings we received from the transformer supplier had some very prominent notes and callouts regarding the "no-iron" zone.

Fiberglass rebar is available - not as strong as steel, so it would have a larger cross-section.

 

[ScottyUK]

Proper high current stuff often runs inside an isolated phase bus duct which keeps the external magnetic field to a minimal level, except for the areas around the bushing turrets where the external field can get pretty high. In those areas the 'no iron' rule is a good plan. IPB is bulky and relatively expensive though, so if a manufacturer can get by without it they probably will.

----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[bdn2004]

ScottyUK,

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.

 

[ScottyUK]

Sounds like a short connection between the transformer and rectfier - we tend to see IPB on runs of maybe 40' or so between generator and GSU transformer. I doubt it would even be practical to use it for a very short distance.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[jkristinn]

bdn2004: would you care to share which isolation material is to be used in the isolation pad you mention?

 

[bdn2004]

Jkristinn,

The isolation material is a trademarked product called Inbord-E. It's described as a paper laminate with melamine surface.