Transformer Hookup

[bakerbrotherselectric]

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?

[Pierre C Belarge]

Some transformers are Step-Down as you are used to and some are Step-Up. If a building is supplied with 208 and they want 277 lighting and/or motors, a Step-up transformer is provided. Some grids/areas do not supply 277.

[winnie]

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

[bakerbrotherselectric]

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.

[Minuteman]

Wired one just as you described for a machine used in a Oil Tool shop. Works great!

[bakerbrotherselectric]

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.

[hardworkingstiff]

.....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.

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.

[zog]

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?

[cschmid]

we are guessing here and using straight theory and we all want to know what the name plate says..please tell use more about the transformer..

[Jljohnson]

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

[winnie]

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

[K2500]

Ground fault detection is probably required.

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

[fordaputz]

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.

[bakerbrotherselectric]

Thanks for all the info everyone, I do appreciate it greatly. I'm going to get the info off of the nameplate to make sure the manuf. allows reverse hookup.

[iwire]

It seems to me that something is just not right with no connection to the XO of a Wye connection.

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.

[LarryFine]

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.
So what happens if a phase opens with the X-0 floating?

[iwire]

So what happens if a phase opens with the X-0 floating?

I imagine you lose a phase on the output.

[e57]

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.

Bob - do you mean 208 wye - connected as delta? No noodle.

[iwire]

Bob - do you mean 208 wye - connected as delta? No noodle.

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.

[cschmid]

I would like more info on the transformer because what he is after is not a regular transformer wired in reverse but an actual step up transformer..

[quogueelectric]

If you want to know if there is an internal ground reference just put a voltmeter from secondary phase to ground and see what it reads. If it is a wye to wye with the secondary being used as a delta the secondary winding will already have a ground reference through the primary neutral.

[don_resqcapt19]

quoqueeelectric,
If you want to know if there is an internal ground reference just put a voltmeter from secondary phase to ground and see what it reads.
If you use a high impedance input volt meter (most digital meters are high impedance) you will see voltage as a result of capacitive coupling. In many cases this voltage will be that same as you would expect to see on a grounded system.
f it is a wye to wye with the secondary being used as a delta the secondary winding will already have a ground reference through the primary neutral.
A grounding connection on the primary side does not create a ground reference on the secondary side. There is no electrical connection between the primary and secondary.
Don

[quogueelectric]

If the xformer in question is one of the most common wye to wye the center tap of both wye configuration are common inherently so if you ground it you are gounding both. Also there is nothing wrong with using a high impedence meter as long as you know how to use it correctly Wiggins were great 30 yrs ago before the computer was introduced. Now it is a dinosaur which belongs in the tar pits for modern electrical work. Unless you want to carry more than 1 meter which is just silly.

[don_resqcapt19]

quoqueelectric,
Also there is nothing wrong with using a high impedence meter as long as you know how to use it correctly
If you use a high impedance input meter on an ungrounded delta system and take your measurements at the line side of the secondary OCPD, with the OCPD open, you will often read exactly the same voltages as on a grounded wye system.
If the xformer in question is one of the most common wye to wye the center tap of both wye configuration are common inherently so if you ground it you are gounding both.
Can you provide an manufacturer's link to such a transformer?...I have never seen one like that.
Don

[LarryFine]

Also there is nothing wrong with using a high impedence meter as long as you know how to use it correctly Wiggins were great 30 yrs ago before the computer was introduced. Now it is a dinosaur which belongs in the tar pits for modern electrical work. Unless you want to carry more than 1 meter which is just silly.
If I could carry only one meter, it would be my Knopp K-60. Its readings are often more usable than a volt-meter's for most trouble-shooting.

Detecting the real presence of power is often more important than knowing exact voltage, especially if you might be reading induced voltage.

[quogueelectric]

If I could carry only one meter, it would be my Knopp K-60. Its readings are often more usable than a volt-meter's for most trouble-shooting.

Detecting the real presence of power is often more important than knowing exact voltage, especially if you might be reading induced voltage.
Real power is also low voltage signal power it is all real power and all follows ohms law. If you feel you need a load carry a few resistors with you.

[quogueelectric]

quoqueelectric,

Can you provide an manufacturer's link to such a transformer?...I have never seen one like that.
Don
I have been working insane hours and have had a brainfart the most common is a wye delta and usually the service is 480 wye grounded at the service and the 120/208 is a sds. The reverse situation does leave you with an ungrounded hv delta and I guess you could corner ground it yet that is as confusing as an ungrounded wye. I have only seenone of each situation in 30 yrs. Once a corner ground in a lipa power plant 480 v distribution where the gen leg grounded out so the 5000 amp buss read 0 volts to ground yet 480 phase to phase. freaky. And secondly an ungrounded sds on a hospital emergency feed which did not read voltage to ground yet phase to phase was 480.