Step up transformer

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pierre

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Westchester County, New York
I have been asked if I thought there would be any problems with installing a step down transformer used as a step up transformer. There is a 208y/120 volt system, and the contractor needs 480y/277 volt for some equipment. The manufacturer told him to wire the transformer 'backwards'.
I thought that I read about some inherent problem with this type of installation. But rack my brain as much as I can, I do not remember.

Does anyone know of any problems with this type of installation?
 
Re: Step up transformer

I have never seen a transformer that has a 480/277 wye primary and a 208/120 wye secondary. The most common distribution transformer has a 480 delta primary and a 280/120 wye secondary. Most manufactures say that you can connect their transformers in reverse to get 480 from a 208 distribution system. When using the transformer this way nothing should be connected to XO terminal. On the 480 side you can either corner ground it or leave it as an ungrounded system. If 277 is needed then this will not work since the 480 sided of the transformer is a delta configuration. 208 delta to 480/277 wye transformers are available but often have a lead time of several weeks.
 
Re: Step up transformer

Normally transformers are 480v delta-208y/120 where the X0 is bonded to the enclosure. If you reverse feed them you unbond that X0 and simply feed it L-L-L to X1-X2-X3 with no connection to the X0 and ground the enclosure.
But it leaves you with a delta secondary which may or may not be an issue. Hopefully you won't need a neutral.
D-Y transformers have 3-legged cores where the Y-Y transformer as commonly manufacture by most manufactures has a 5-legged core. As I recall Westinghouse did have a proprietary 4-legged core which C-H may now have.
Y-Y designs must address something called Ferroresonance and imbalance which really aren't an issue with the 3-legged design used with D-Ys.
 
Re: Step up transformer

I ran into this same sort of problem with a 480 step down hooked up backwards,this was an existing installation.
I called the manufacturer (sq d ) and asked the engineer if this was kosher.
The engineer said although it is commonly done in the field they would not honor any warranties because it is?not hooked up as per name plate.

The reason I called is because I could not get a reading to ground from the 480 volt side even though the case was solidly grounded and the xo was not connected on the 120 volt side.

I removed that transformer and put in a transformer designed for step up.
 
Re: Step up transformer

I've asked this of the local Cutler Hammer and GE reps at meetings. Since I don't run into this I've forgotten myself. I do remember a reduction in useable KVA, like about 65% that had something to do with what happens when the coils magnetized.
I have an email for the local Cutler Hammer rep if you would like it, also a phone number.
 
Re: Step up transformer

There is NO reduction in KVA.
With regards to grounding the 480 side, that is not having a ground, of course it's not grounded. There is no neutral to ground because it's a delta and , as such, you must consider corner grounding the delta as a grounding 'B' phase which is no big deal. You have to do it.
Just an editorial comment: I believe you are going to find that there is less and less commitment product and application help available from the distributors as well as the manufactures. They don't have either good product knowledge or the experience and it's getting worse. Most are going to something called knowledge based software which provides black and white answers for questions. The questions is entered on the computer and up pops the answer.
Accurate answers to true applications questions are going to be harder and harder to get.
 
Re: Step up transformer

My co-worker did this, but he subjected himself to only 3 phase loads, and not single phases loads. I would not recommend this whatsoever. I would just use a transformer which steps up the voltage, and not worry about wiring it backwords. JMO!

Question, it becomes a corner-ground delta, how? Sorry for the naive question. :)
 
Re: Step up transformer

Lady, Is your question "it becomes a corner-ground delta, how?" in response to my statement
"you must consider corner grounding the delta as a grounding 'B' phase which is no big deal"?
Please note that if you use a common D-Y transformer to step that the delta doesn't automatically "become" a corner grounded delta. It is the installer that must intentionally corner ground it he finds that it is the best way (safest?) to distribute the delta.
Also, transformers don't care which way they are fed, either high voltage to low voltage or low voltage to high voltage. Since they are normally used to step down a distribution voltage we view them as being reverse fed when we use them to step up a voltage.
In addition, a couple of interesting questions would be:
Are transformers marked as having a primary and secondary and/or high voltage and low voltage? Are transformers wound as 208v delta-480y/277 marked any differently than a 480v delta-208y/120 to distinguish the side being feed?
I personally believe that we assume the HV side is the side that is being fed.
Another interesting point s that step up transformers may not be as special as one might think. There are 208d-460y/266v and 230d-460y/266v drive isolations transformers designs commonly available.
 
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