OK, I'm fishing for ideas, and experience, for an idea of mine.
The site is a massive industrial facility, with approximately 700 light fixtures; lights are 400W MH for the most part. Plant is fed by a 480v ungrounded delta system. Building is of the quickly errected I-beam frame / sheet metal skin type.
As you might guess, there are power quality issues. One bad ballast, and the induction equipment starts making bad welds.
I'd like to propose that we install transformer(s) to convert the power supply to a grounded 'wye' arrangement, using the building steel as my transformer ground.
Any thought, or experiences you'd like to share?
I'm guessing that running a ground wire from the transformer to the outside is a problem, or you would use a ground rod and bond the building.
Is there any compelling reason why this is ungrounded? Easier might be to get the PoCo to retap their transformers to wye if this will work for the customer.
You are also going to have to pick up the metal water pipe in your ground electrode system if it is present.
Are you sure it isn't a 277/480 Wye at the service and the neutral is not grounded?
Can a unloaded 3 phase transformer with a 277/480 star winding be connected to create the neutral?
If the Poco transformer wired Wye they will demand the neutral be brought in as a matter of course.
I've never seen it omitted.
But, I haven't seen everything.
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Reno: the transformer that you're in need of is a ZigZag.
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You're also telling the tale of woe that caused EUSERC to entirely abandon delta Services -- unless there's a compelling reason.
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375 Watt lamps are now available that produce the same lumens as 400 Watt classics.
The economics are astoundingly favorable for swapping over ASAP.
Larry, if it's star wound -- there's your neutral tap: Xo.
If it's delta wound -- a zigzag transformer is used to establish a high impedance ground -- this is sometimes done in industry when the better traits of wye transformers are needed at the same time the system has to continue running in the event of a ground fault.
( Think paper mills, etc. )
The plant ... this is a major, heavy industrial complex ... was built in the early 1990's, with an emphasis on CHEAP.
The PoCo delivers high voltage; the customer owns the transformers that step it down to the ungrounded 480 delta. There is no compelling reason, apart from they wanted to save money by not pulling a neutral.
FWIW, my input two years ago led, last summer, to a 'pilot program' where a small section of the complex replaced the HV transformers with WYE transformers.
The equipment in this plant is far older than the plant itself; some of the production equipment was used in WW2 to make cannon barrels. probably not an issue then; ungrounded delta works fine when all you have are motor and resistance loads.
Of course, these days we have frequency drives all over the place, and we heat the metal by induction. 1200 amps of Scr's firing will put one hell of a spike on the line.
Ironically, many at the plant still think it's the PoCo that's giving us "bad power."
High impedence ground ... that's exactly what we had when that ballast shorted out, causing bad welds on the line.
I'm not done with my daydreams, with just a power conversion. I figure separating the lights from the production power is just the beginning. (Ambitious, aren't I?)
I'd like to deep-six the MH in favor of T-5's ... which ought to help the summer heat load and cut the power use in half. Get better light in the bargain as well.
Another advantage to switching to 277 is that I can get a 277v multi-tap ballast anywhere; 480v ballasts are harder to find, and thus much more expensive.
I am aware of the grounding issues. Oddly enough, the buildings are extensively grounded. Remember, just because the system is 'ungrounded' it does not mean the building need not be. After all, the system relies upon building bonding to make the fault monitors work. Plus, of course, there is that incidental 120v for the receptacles.
The place has a "grounding guru" who has been kept apart from me ... but it's clear his reputation was built on the results he produced by making sure things are bonded.
As for an 'earth ground' ... well, I submit that the cement slab, with all the bond wires and steel beam anchors, cannot help but be one heck of a big Ufer. Grounding doesn't get much better than that.
The issue about grounding is compliance with the rules. If the rules say you can use the building, then you can. But, if they did, you probably wouldn't mention it here. So, what do the rules say?
"The rules" are pretty clear.
The NEC has allowed use of building steel for grounding transformer windings for ... well, as far as I know.
Considering the building steel, and the usual rebar as a "Ufer" or concrete-encased electrode was something of a debateable item, until the 2011 code language made clear that it was acceptable, even with only the 'usual' tie wires.
Some concerns have been raised (in other discussions) regarding building steel that isn't in contact with other steel, but might become 'hot.' That issue really isn't relevant here; I defy anyone to connect structural iron with multiple 1-1/4" bolts and NOT have them bonded well enough to sneeze at a lightning bolt- and that's without considering the massive bare copper wires we have in this building, exothermically welded to the various columns.
I have encountered some confusion over the years, as to exactly what one accomplished by 'grounding' a service. What's important to recognize here is that the electricity from the secondary of that transformer has only one place it wants to go: back to that transformer. It doesn't want to go to the PoCo, into Mother Earth, or anywhere else. That's the concept at the heart of 'separately derived systems.'
What bonding the various systems together accomplishes is that it ensures that all the 'neutrals' are at the same potential, relative to each other. That's a direct result of our having the neutrals bonded to the grounds; our neutrals may be 'isolated,' but the grounds are not ... so the 'separate' systems are actually indirectly connected.
A lot has been written about what neutrals do, and why we ground them. While we can all understand fault clearing and static dissipation, the other reasons are not so well understood. You'll hear things like 'provides a ground reference' or 'stabilizes voltage swings in response to loads,' but these concepts are not nearly as well understood as the fancy words might lead you to expect. This is why, IMO, Article 250 of the NEC has been in such great need of serious editing for decades. Code language there is often derived from concepts that have long since been abandoned. The past few code cycles have done a great deal to clear things up.
Corner ground it and tape all the grounded conductors white.
What I have seen, the way to avoid problems with equipment in a large plant is to install isolation transformers for the electronic stuff and NOT ground those transformers. I didn't pay any attention to whether they were ordinary transformers or iron rich or iron poor. Aren't welders transformers, anyway? As you might tell from my previous comments, we can't bond to the building, so I have no experience with that.
As you might guess, there are power quality issues. One bad ballast, and the induction equipment starts making bad welds.
How does a bad ballast lead to bad welds?
A bad ballast has the effect of 'grounding' one leg of the 'ungrounded' delta. If there's another incidental ground somewhere, you have a fault. If the fault is a high-impedence type of fault, you don't trip breakers or light up the pahse monitors ... but your line voltages swing all over the place. That, in turn, means that the electronics of the induction equipment don't work right.
For those unfamiliar with 'induction welding,' that is a process quite a bit different from anything you'll see in a workshop. There is no electrode, rod, or flux. Instead, steel is passed through a magnetic field that cycles very fast .... we're talking about thousands of cycles per second. This filed makes the metal withing it red-hot. The red-hot metal is then squeezed together, where it mixes with the other red-hot metal and forms a weld. This is how pipe is made (including EMT).
The induction welding equipment has no transformers; it relies upon very rapid switching of the AC supply to create both the high frequency and the higher voltages used. If your AC supply coming in is not a clean waveform, the electronics (SCR's and IGBT's most often) won't work right.
Of course, the opposite also applies: the induction equipment does introduce harmonics to the AC supply. Not as bad as a variable speed drive, but there is the effect.
Hence, my desire to isolate the lighting from the power.
Perhaps a better link, that explains some of the issues I am facing, is:
http://www.iaei.org/magazine/2010/03/are-we-really-ungrounded/
A corner ground won't work for you? You won't need any additional wire beyond the GEC.
This requires some awareness about what is going on but it is a fairly simple way to ground a delta where you are not using any line to neutral loads.
Another option might be to just add a 480 wye transformer for the loads that are not going to the welders. (maybe they expect an ungrounded source) I assume they already have a 120/240 or 120/208 transformer for the office loads.
Just had a visit today by a rather sceptical gent, rep of a major French/German maker of all manner of power related equipment.
End result? My description of the existing equipment is accurate, and the idea should work. Quote will follow.
A few matters that were clarified:
1) This is an instance where an 'ordinary' transformer will probably suffice. No real need for a "K" rating, since the lights are not the source of the 'harmonics.' Here it is important to recognize that there's more to 'power quality' than just harmonics; and,
2) Yup, they can make a lot of stuff that's not in the catalog. For example, the transformers I want in an unvented (though still Nema-1) case. That will be enough to keep the rather modest amount of conductive dust at bay.
Of course, now I need to dig deeper into the plant's distribution system. This is going to be an incremental project.
Search "induction welding" on Youtube. Fun stuff.