In reviewing some field notes and pictures from a plant where we are doing some unrelated control system work I noted that the site's existing 1200A service (480V) appears to be ungrounded wye. The plant operator had noted to me in a visit that the electrical dist. was ungrounded delta, and I had not looked closely at the connections on the outdoor transformers when on-site. I have seen ungrounded delta systems with Ground detection systems but I don't think I've seen ungrounded wye - is this a common installation?
Site is an industrial utility plant, some large motor loads, probably 50 yrs old. Fed from (3) 1 ph. transformers connected delta primary. They do have indicating lights installed on each incoming phase for monitoring loss of phase but don't have any alarming or any of the markings identified in 250.21(C). At least not from what I can see in pictures.
I have reviewed the images you posted of the transformer bank. The primary is wired as a grounded wye but secondary side is wye but ungrounded. It seems somebody has omitted the grounding of the paralleled secondary side, but you have to remember, it will function like this without a neutral, however to make accommodations for updated controls and other devices, such as ground fault breakers and specific new instrumentation, since they require a neutral and ground wire, which forces engineering to add a ground to the unused side of the coils, as well, supply neutral to the inner panels the customer needs replaced, etc. Without having the neutral won't change anything electrically, for the function of the existing components, but will be needed to operate anything that runs from a grounded wye system (using the neutral)
I saw another post regarding that the wye is the same as delta without the neutral. That is far from the truth, the delta returns from other phases through another phase. The WYE or STAR connects one end of all three coils together and feeds the phases with the other three. If the three bushings for neutral are not connected to a load, the current will return back through another phase giving much different results, it gives each phase the ability to return an imbalance through another phase instead of through a neutral wire. The ground just ensures casings are bonded and of they aren't the manufacturer provides ground straps either internally or externally on transformers.
Before anybody continues, the power should have been shut down from the power company or customer owned aerial cutouts, if they exist, then test for potential to ensure there is no generation backfeed or induction, then if ok, makesafe such as grounding of all primary and secondary phase bushings.. Then remove tops of transformers to see if grounding straps have been connected to any of the bushings that would normally ground the paralleled secondaries that normally would be connected to the neutral conductor. To continue and put in what you need to do the work... (adding neutral wire, ground wire and conduit between the transformer pad and the interior switch panel)
I haven't seen this scenario many times, but I did encounter a neutral free wye generator system built in the 20's by Westinghouse at one of the older dams in CT. It had the ability to add multiple tap resistor neutral taps or it would be run without any neutral as Tesla did with his experimental generation circuits that supposedly would all produce power from hydro generation, but would deliver 11 kV of energy through ungrounded, non neutral wye generators which then fed delta transformers for power companies. We are NOT supposed to connect wye systems ungrounded, nor does the NEC allow wye systems to be installed on any new installation without a neutral conductor. As long as they remain a federally funded operation that feeds federally funded electrical transmission systems, they could use whatever means necessary to fill our power lines with electricity that basically costs them nothing to operate other than maintenance of the generators and dams that hold the water to use gravity to generate power.
I can't get into the specifics, but I can say if we did this today in every installation, we would all have no job in the electrical industry, so thank JP Morgan for stopping Tesla otherwise our society would not make money from electricity and no distribution of anything, no wires, no nothing between homes and commercial or industrial systems, no meters, no power companies, etc..
My guess is that the transformers at this company fed a panel, then were possibly stepped down or ran delta machines such as lathes, mills, or other motorized equipment? To understand what is really happening, when the system is forced into upgrade, such as electrical metering, and a grounded wye system with neutral added, My bet is their electric bill will go up substantially, maybe even triple in cost.
This has happened to us in the past when one of our customers, who is actually one of the top prep schools in the country have upgraded from an ungrounded, neutral free wye system to a brand new delta/wye system with padmounted transformers that use grounded neutrals, and even after having electrical testing to verify the power companies new meters were accurate, the campus's electric bill went up from 13k per month to almost 40k per month.
The system installed is perfectly operational today but the change in utility transformers, electrical metering, and of course how they were wired is what cost them the difference since they were using less electricity but getting more for some reason for many years. This effect is rarely documented but I am a witness of this actually happening and many others in the industry right here in New England.
My statement to you is that I would have them replace what they could to remain operational and don't change much so they don't pay more, but, this does endanger our industry.