# Backfeeding Delta-Y Transformer to Y-Delta



## petedrt (Jun 29, 2016)

I need to step up 208Y/120 to 480 Delta. 

The facilities power has been stepped down with a 480Delta Primary to 208Y/120 Secondary, and my task is stepping the voltage back up to 480 Delta (to be used for shore power on vessels in the Dock). 

I have not found any available step up transformer's with this configuration, therefore I am interesting in backfeeding another 480-208Y/120 transformer using the primary as secondary and visa versa. I know this can be done but my concern lies within how to connect the neutral on the Y (now the primary) in a way that does not cause any shorting issues or current inrush. 

Short of ordering a special order transformer for this application, does anyone have any advice on how to make the connections or alter the transformer to function correctly. Additionally I am concerned about what to expect in terms of phase shift. Should I expect any issues, or being that the voltage goes from delta-Y back to Y-delta, would this essentially return the voltage back to its original phase? i.e. +30 degrees then -30 degrees...?

Any input would be appreciated regarding this issue. Thanks


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## papaotis (Jun 8, 2013)

not my forte, but any chance of just skipping the xformer altogether? sounds logical but then theres always those missing details!


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## frenchelectrican (Mar 15, 2007)

Is a way you can elemated the transformer for this circuit and is a spare conduit ya can use ?


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## Aegis (Mar 18, 2011)

Just my own thoughts so don't listen to me lol, but there's no reason for the wye to be connected, the 120v isn't doing anything.

208 on the Wye lugs, 480 on the Delta.


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## petedrt (Jun 29, 2016)

That was my first thought, but cannot be done in this situation. Due to construction there is no way for me to use 480 from the source and run lines. Sorry I should have listed that important detail.


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## petedrt (Jun 29, 2016)

Just want to make sure I am understanding correctly, and providing enough detail on this. 

The Port facility uses 300 kVA on 208Y/120, and a decent amount of it. Due to construction there are no longer 480V lines running to the Dock for shore Power. If I backfeed a step down to be used as step up @30kVA, and connect all terminals the same way (but in reverse) are you saying that I am not going to have any issues because the 120v isn't doing anything?


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## Aegis (Mar 18, 2011)

It's almost like a delta delta transformer, only the primary is a Wye.


Tap off of that 208v 300KVA service and add a fused disconnect or breaker. Then from that run the 208v 3 phase wires to the 30KVA transformer and step it up to 480v.


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## frenchelectrican (Mar 15, 2007)

Ok now i can see what you mean on that ...

Yes you can reverse the ∆-Y transformer however ...

Remove the XO bonding and ground the ∆ B phase ( 480 volt side ) then it will work..

Make sure you post a note a corner grounded system so other will know.


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## papaotis (Jun 8, 2013)

hmm, the missing details spew forth!


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## frenchelectrican (Mar 15, 2007)

papaotis said:


> hmm, the missing details spew forth!


Who is missing details ?


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## Southeast Power (Jan 18, 2009)

petedrt said:


> I need to step up 208Y/120 to 480 Delta.
> 
> The facilities power has been stepped down with a 480Delta Primary to 208Y/120 Secondary, and my task is stepping the voltage back up to 480 Delta (to be used for shore power on vessels in the Dock).
> 
> ...


Reversing a wye/ delta is done all of the time. I always find it to be cringe worthy for some reason. 
For one thing, it's just goes against everything electricians learn in the field when you connect that phase to ground and turn it on.
Second, some people don't connect a corner and have no ground detector set up.

You are going to find the voltage a bit low when you are done so, be ready to move those taps.
Also, Marc mentioned connecting one phase to ground or corner grounding the transformer. This used to be optional and considering you mentioned the work "dock" I think boats don't do that, they use ground detectors. Just be prepared for that if the boat rejects the shore power.
Also, you are best to just leave Wye side XO alone and whatever you do, do not connect anything on the delta side to it.

You also mentioned a 30kva transformer, just remember, nothing is free. You need twice the amperage on the line side. Don't expect more than 35 amps or so out of that little fella.


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## Aegis (Mar 18, 2011)

When I've hooked up a delta delta before, all I put in was the ground fault indicator lights and the inspector was fine with that, no grounding. Is grounding one of the legs on the secondary delta mandatory? I'm also in Ontario so maybe things are just done differently.


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## Southeast Power (Jan 18, 2009)

I think best case is to go delta/delta but, special order for sure. 
everybody can get their hands on a delta/ wye with a single phone call and have one in an hour or might be sitting on a few surplus ones.


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## Big John (May 23, 2010)

This is entirely doable. 

Leave your 208 wye neutral disconnected. Do not hook any conductors to this or you'll develop ground loops.

Corner ground your 480V delta to stabilize that voltage.

You will likely have higher inrush than on a delta, so you may trip primary protection.

You will have a phase shift, look at your nameplate vector diagram, it will show you how much. Not sure why this would be a concern unless you're paralleling.

My only worry is your 480V grounding. I've worked on a couple ships and they've had screwball distribution: Floating and weird center tap grounds. 

I would be concerned about feeding ships with the 480V-to-ground you will make from a corner-delta. I would run that by somebody who knows about ship power.


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## petedrt (Jun 29, 2016)

Gentlemen,

Thank you all for your input on this. It definitely points me in the right direction and has brought up things I hadn't considered thus far. 

This is my first time using this site and has been very helpful.

I appreciate it.


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## Aegis (Mar 18, 2011)

petedrt said:


> Gentlemen, Thank you all for your input on this. It definitely points me in the right direction and has brought up things I hadn't considered thus far. This is my first time using this site and has been very helpful. I appreciate it.


No problem, I'm sure you'll be able to help us in the future at some point


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