Transformer Hookup

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bakerbrotherselectric

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Installed a 112.5 KVA transformer this past weekend as directed. Went from 120/208 to 277/480. I've installed plenty of transformers in my short career(10 yrs) but, I've always gone from 480 to 120/208. This situation called for the opposite. Primary on the X1 X2 X3 and the Secondary on the H1 H2 H3. The equipment did not require a neutral.Will the transformer work correctly either way it is hooked up? Are they designed to be installed this way?
 
Transformers by their very nature 'work' in either direction, but there are specific design issues that can be problematic when being used as you describe.

You say '120/208 to 277/480' and 'Primary on the X1 X2 X3 and the Secondary on the H1 H2 H3 '.

But what you don't say is if you have a secondary neutral (H0), and if you grounded that neutral. You also don't say the primary or secondary are wye or delta.

If you've taken a common 480V to 208/120V step down transformer and wired it in reverse, then you _may_ have violated the listing of the transformer, and you've almost certainly introduced significant grounding issues that need to be carefully considered.

-Jon
 
there was no HO. The equipment did not require a neutral. We did pull a ground in with the feeder wires as well a bare ufer ground that was bonded to the transformer. A ground wire was also pulled to the equipment, that too was landed in the transformer. Nothing was landed to the XO.
Delta Primary Wye Secondary.
Your typical transformer
H1 H2 H3

X1 X2 X3
X0

Wired it up Primary to the X's and Secondary to the H's. So it ended up Wye primary Delta Secondary.
 
this one worked great as well, under no load though. There molding equipment wasn't 100% together, so I opened the disconnect and the main breaker. Surprisingly the transformer was very quiet. Wasn't expecting it to be that quiet.
 
bakerbrotherselectric said:
.....Nothing was landed to the XO.


Wired it up Primary to the X's and Secondary to the H's. So it ended up Wye primary Delta Secondary.
Click to expand...

I sure hope a strong transformer person comments on this. It seems to me that something is just not right with no connection to the XO of a Wye connection.
 
Your protection on the "new" primary side may not function properly and I wonder what was done with the nuetral on the "new" secondary side? Was a coner grouded delta configuration used?
 
There are several fairly long posts that talk about this exact issue in the archives as well. Type "Reverse Fed Transformer" in the forum search bar and take a look at some of the threads that you find. The most important issue that you need to carefully consider is the grounding (and lack of bonding XO) when a transformer is used in this manner.
Happy reading
 
As mentioned above, there have been other threads on this subject that go into detail. You should take the time to use the search feature to find them. Here are two that popped up when I searched for "transformer reverse":
http://forums.mikeholt.com/showthread.php?t=87677
http://forums.mikeholt.com/showthread.php?t=86269

In a nutshell:
you _don't_ connect X0 when the wye is on the primary side.
you _do not_ bond X0 to ground when the wye is the primary.
a reverse connected transformer won't have taps on the primary side, so you can't adjust for incoming voltage
as an SDS, the transformer _secondary_ needs to be bonded to ground, or you have an 'ungrounded delta'.

As you've described the setup, the 480V going into your equipment is _ungrounded_. If you have a ground fault in your equipment, no breakers will trip, not until you have a _second_ ground fault. Ground fault detection is probably required.

You can ground your secondary, but this will require 'corner grounding', or will require a 'zig-zag' transformer to derive a neutral to ground.

-Jon
 
winnie said:
Ground fault detection is probably required.
Click to expand...

Just for reference, depending on the OP's application, transformer listing aside. See 250.20(D)(E); (250.21) for ground fault detection requirements.
 
208 to 480, step up.

First the Xs will always be the 208 side since they mean LOW VOLTAGE (Not primary) that being the case the transformer might be design to go 208 up to 480.

Most of the replies however assume the unit is a 480 to 208Y/120 being BACKFEED - which is allow only if the nameplate does not say PRI: 480 and SEC: 208Y/120. (Note if the nameplate states Pri/Sec - most manufacure allow the backfeeding via some other note on the device or instuctions) So we now have a 4-Wire system feeding a WYE-Delta Transformer and need to be concern about the following:

If the primary service is wye connected 4-wire, connecting the service neutral into a wye delta connected transformer it causes a different set of problems:
(1) Services typically become somewhat distorted in both voltage and phase
angle because of the influence of both unbalanced loads, and unequal
power factor loads. By connecting the neutral to the wye side of a wyedelta
connection, the wye side attempts to conform to the phase angles and
voltage levels of the service. These voltages and angles are transferred
magnetically into the delta secondary. The simplest explanation of the
consequence of this is that the delta secondary must follow the rules of
triangles, i.e. all three corner angles must add to 180 degrees. Primary
service distortion may not provide phase angles on the secondary that
follow these rules. That circumstance will cause a single phase circulation
current to flow in the delta, which will reflect as extra line and neutral
current in the primary side. In strongly distorted systems, this additional
current can seriously derate, or even overload the transformer.

(2) Perhaps more serious consequence of connecting the neutral on the
primary occurs when a phase is lost on the primary service. In that
scenario, the delta connection will energize the leg of the primary that is
connected to the ?dead? line, causing it to supply any loads that may be
connected to that line, either upstream or downstream. In addition, it
?back-feeds? the open service line, creating a potential hazard to
maintenance personnel.
 
hardworkingstiff said:
It seems to me that something is just not right with no connection to the XO of a Wye connection.
Click to expand...

It should not be connected.

We are talking about a 480 Delta - 208 Wye/120 transformer being used in reverse.

You can only get 480 out of the 'H' connections and you would only supply X1, X2 and X3 with 208 delta, no connection at all to XO.

If you did connect XO to the 208 systems neutral or ground and then lost one 208 supply phase the conductor on the neutral can be overloaded as the transformer will continue to try to supply the 480 using two phases and the conductor on XO.
 
iwire said:
If you did connect XO to the 208 systems neutral or ground and then lost one 208 supply phase the conductor on the neutral can be overloaded as the transformer will continue to try to supply the 480 using two phases and the conductor on XO.
Click to expand...
So what happens if a phase opens with the X-0 floating?
 
iwire said:
It should not be connected.

We are talking about a 480 Delta - 208 Wye/120 transformer being used in reverse.

You can only get 480 out of the 'H' connections and you would only supply X1, X2 and X3 with 208 delta, no connection at all to XO.

If you did connect XO to the 208 systems neutral or ground and then lost one 208 supply phase the conductor on the neutral can be overloaded as the transformer will continue to try to supply the 480 using two phases and the conductor on XO.
Click to expand...

Bob - do you mean 208 wye - connected as delta? No noodle.
 
e57 said:
Bob - do you mean 208 wye - connected as delta? No noodle.
Click to expand...

Normally this transformer will be called a 480 Delta - 208Y/120. However it will, at least in my experiance be supplied from a Wye service. IMO the fact that the service has a neutral (is a Wye) does not change the fact the feeder is supplying the transformer in a Delta.
 
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