My company is designing the electrical system for an industrial installation. The utility company is providing the transformers. It is standard for them to have a delta to wye configuration however my motors do not require a neutral. I bounced the idea off them to provide delta to delta transformers, which they will but if a transformer goes down they wont be able to instantly replace it. With the delta to wye configuration the plant wont stay down but for a few hours, obviously preferable from an owners standpoint.
Is there anything wrong with providing a delta to wye transformer and just not use the neutral?
Assumptions:
This is industrial ("motors" comment),
And it is 480V, 3ph (you never verified)
Yes, the non-standard xfm is a real problem. Two of my customers opted to spend ~$2M each to have a spare on hand - choke... that's definitely some real money.
You already have good responses on using a wye grounded or ungrounded. And un-grounded Delta is hard to handle. The voltage to ground drifts around, suspectable to lightning surges, and highly suspectable to arcing, restriking ground faults. Not one of my favorites. Only use it if equipment requires, and keep it really short.
There are a couple of other things to consider.
- What level of power are you looking at? Different design criteria if the system is 500KW than if it is 1MW, or if 5MW.
- Are you looking at ungrounded systems to save a few dollars on copper? Or is this for continuty of power issues?
If the issue is the copper neutral cost, I won't have any help beyond what has already been said. Bolt the main to the xfm, run the neuetral to the main - and no farther. If not:
At 500Kw, grounded Wye is pretty easy to handle. SCC is ~ 12KA. Equipment costs are low, grounded Wye is simple, well understood, and easy to T-shoot. 277V is available for lighting, but you will still need 480/208/120 xfm for hotel loads. again, you can put the main as close as you want to the xfm and the neutral has to go no farther.
At 1MW and up, I would seriously consider HRG. Utilities don't normally supply HRG, but they also don't usually want to supply un-grounded systems. If they will supply an un-grounded wye, ask if they will use the same transformer installed as an HRG. If it will work for one, it will work for the other. Offer to buy the resistor and if it bothers you, stock a spare (although I have never heard of one failing - except from a ground fault caused by a backhoe driving one into the ground):roll:
Set the resistor at (or on) the xfm. The neutral does not have to go past the resistor. Just take the three phases and the grounding/bonding conductor to the the switchgear.
HRG has a lot of advantages. Phase to ground faults are really damaging on grounded systems - and these go away. Phase to phase faults clear quickly. Continuity of service is good - the plant will stay running with one ground fault. But one has to go find it before too long.
Can't use 277V lighting, so one can either scatter a few small 480D/480Y xfm around for lighting or run the lighting from the 208/120 hotel load xfms - minor design decisions.
Up in the 5MW range you may want to consider buying the xfm and taking the service at 4160V or 13.8KV, or 69kV. This is a whole new set of design issues.
Give us a bit more on what size you ar looking at
ice