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.
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.
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.
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 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.