I am volunteering at a large camp that an non "electrical" person designed using a couple of DIY books. He reverse fed transformers from 480v Y to 600v delta to save wire sizes over 1000' runs. I am concerned about a lot of things, but first wondering before I pull covers what happens to Aic when you reverse feed? Appears secondary system would have to be ungrounded delta though not labeled as such.
Reverse fed transformers
- Thread starter sculler
- Start date
- Status
- Location
- Massachusetts
It could be either corner grounded delta or ungrounded delta.
Without further info there is nothing that jumps out as being wrong with this set up.
kwired
Electron manager
- Location
- NE Nebraska
If ungrounded - needs ground fault detection/indication system.
Otherwise it can be corner grounded.
I first question how many DIY books are out there that address 480 or 600 volts three phase applications. I suppose there are some but those are probably a drop in a bucket compared to how many are geared to residential DIY wiring.
I also have to question just how much cost is saved stepping up from 480 to 600 volts when you consider the current difference isn't all that much. Transformers and needed accessories would need to cost less then the only slight change necessary in conductor size for most things. We run ~ 1/4 mile runs quite frequently in this area to 50 - 100 HP @ 480 volts irrigation well motors - because it costs less to upsize the conductor a size or two then to decrease it only a size or two AND then add a transformer (or two) on top of it.
Otherwise it can be corner grounded.
I first question how many DIY books are out there that address 480 or 600 volts three phase applications. I suppose there are some but those are probably a drop in a bucket compared to how many are geared to residential DIY wiring.
I also have to question just how much cost is saved stepping up from 480 to 600 volts when you consider the current difference isn't all that much. Transformers and needed accessories would need to cost less then the only slight change necessary in conductor size for most things. We run ~ 1/4 mile runs quite frequently in this area to 50 - 100 HP @ 480 volts irrigation well motors - because it costs less to upsize the conductor a size or two then to decrease it only a size or two AND then add a transformer (or two) on top of it.
Good points if I may say so.
I wondered about the economics too. And what, if anything, would you use 600V for? Maybe the camp is in Canada.....
kwired
Electron manager
- Location
- NE Nebraska
I was kind of presuming it got stepped back to 480 volts, but seems you would spend less just increasing the conductors a size or two and get nearly same performance at less cost then purchasing two transformers, and associated items necessary for those transformers.
If not stepping back down then the load being supplied needs to be designed for 600 volts - not very common at all in the US, and again probably will cost more then more commonly available 480 volt items.
GeorgeB
ElectroHydraulics engineer (retired)
- Location
- Greenville SC
- Occupation
- Retired
The textile and brick industries, in the 30s through the 60s, used 600V (we called it 550) extensively. I'd guess that this amateur obtained some surplus 600:480 transformers from plants of that era.
As recently as 1990, I was designing control panels for fiber handling in textile mills that still used 600V.
The VSD market for 600V drives is very much alive and well down here in the southeastern US.
Maybe sculler will return and tell us a but more about his system.
DIY'r took 600V idea from Canadian project with private hydro project running at 600V Not sure of his economic analysis to do similar in US, but it's here. I'm worried about other issues than motive at this point.
System is not labeled as ungrounded, yet no phase grounded indicators. System steps down from 600V 3 phase at (4) remote 120/240 single phase transformers.
Can anybody answer first question about Aic in step up transformers?
The AIC requirement will be lower at the 600V level.
- Location
- Massachusetts
It does not have to be labeled as grounded or ungrounded
Either system can be installed safely.
OK, confirms my long hand calcs, wasn't sure I did correctly.
My 3 phase delta experience is typically with 3 phase motors and with 3 phase step down Y grounded transformers. In these transformers unbalanced loads go to neutral. Motors typically are balanced close enough there's no issue. But with step up trans with ungrounded delta supplying multiple single phase loads doesn't system end up with circulating currents?
Another concern is insulation rating of secondary conductors, 600v.
If secondary is ungrounded a phase fault will increase voltage on other phases to 830 v exceeding insulation rating. Also seems that it would impact secondary voltages on down stream single phase transformers if one of their primary legs went to 830 v.
Another trick from the DIY'r was to dual feed a transformer. Normal power through trans primary as step down from 600 to 208/120v, but in emergency gen set operation, generator feeds 120/208v y into secondary to feed 600 v 3 phase to facility distribution boards.
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Saturn_Europa
Senior Member
- Location
- Fishing Industry
- Occupation
- Electrician Limited License NC
Please elaborate. I was under the assumption that the greater the voltage the greater the hazard.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If the utilization voltage is 120/240 three wire, it would only require an additional step up transformer at the source end and a different transformer at the load end. That keeps the additional cost of the higher voltage lower than in a step-up/step-down situation.
mobile
It's a matter of what the transformer can deliver under fault conditions.
- Location
- Massachusetts
NEC 250.21 (C) : ""ungrounded systems shall be legibly marked " Ungrounded System""
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Assuming infinite bus worse case
i sc = va/(1.732 x v ll x pu Z)
as v ll increases, i sc must decrease
kwired
Electron manager
- Location
- NE Nebraska
If he starts out with 480 service voltage, how cost effective is it to spend money on a transformer to only boost the voltage to 600 volts vs possibly only needing slight increase in conductor size for the voltage drop if you just supplied the 480 volts? A step up from 120 to 480 over a long distance is going to make more sense then stepping up from 480 to 600.
Peak of the AC voltage? But that is the same whether grounded or not.
- Location
- Massachusetts
- Status