Reverse fed a step down transformer, getting different reading on C phase to ground

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SeanJo

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Location
Houston, TX
A job I am helping on required a step-up transformer due to someone ordering the wrong equipment. The service to the building is 3phase 120/208v Y, and the new CRAC unit requires 480v 3ph. It was calculated that adding a step-up transformer would be cheaper than reordering the new CRAC unit :roll:. After wiring the transformer and starting it up, we tested voltage to be sure everything came out alright. The readings were as follows:

Primary:
A to grnd-120 A to B-208
B to grnd-120 B to C-208
C to grnd-120 A to C-208

Secondary:
A to grnd-277 A to B-480
B to grnd-277 B to C-480
C to grnd-240 A to C-480

It's also worth noting that we placed a jumper from X0 to the transformer grounding lug, even though the loads coming off of the transformer are going to be strictly 3phase Motors. Can anyone enlighten me as to why the C phase to ground voltage would be different when all coils were tapped at the same location(lug 3)? I wish I could have tried swapping 2 of the coils and seeing if it was only an issue with X3, meaning it could be an issue with the transformer itself, but it was the end of a long day and the system will not be started and operational until next week. Anyone with ideas or experience in this type of situation?
 
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SeanJo

Member
Location
Houston, TX
It is actually a wye to wye step down transformer that is being reverse fed to step up the voltage from 120/208 to 277/480 for the sole purpose of supplying power to 3ph motor loads
 

SeanJo

Member
Location
Houston, TX
If this was a delta-wye xfmr, what would be the cause of the c phase producing a lower voltage compared to the other coils when all were wired in the same manner?
 

iceworm

Curmudgeon still using printed IEEE Color Books
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...a step-up transformer ...service to the building is 3phase 120/208v Y, and the new CRAC unit requires 480v 3ph.

...we placed a jumper from X0 to the transformer grounding lug,... . Anyone with ideas or experience in this type of situation?

Couple of curiosity queations:

This step-up xfm, is it a 208D/480Y? Or is it a 208Y/408D?

The Xo terminal, is that connected to the 208 windings or to the 480V windings?

ice
 

coop3339

Senior Member
Location
NJ
If this was a delta-wye xfmr, what would be the cause of the c phase producing a lower voltage compared to the other coils when all were wired in the same manner?

If it is delta connected on the high side there would not be a solid reference to ground on the delta side. What you would be reading is capacitive coupling and that may be why you are getting those readings.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
It is a Primary Delta/Secondary Wye. I can only make an assumption that the X0 terminal should be connected to the secondary windings?
There is no H0 terminal and the X0 terminal is connected to the low voltage windings.
You currently have an ungrounded 480V delta. You can either corner ground it or operate it ungrounded with a ground detector IF the CRAC can accept one of those.
On the input side you would normally leave the X0 wye point totally unconnected. You must ground/bond the transformer case.
You have found one of the drawbacks of reverse wiring a delta to wye transformer.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
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EE (Field - as little design as possible)
It is actually a wye to wye step down transformer that is being reverse fed to step up the voltage from 120/208 to 277/480 for the sole purpose of supplying power to 3ph motor loads

It is a Primary Delta/Secondary Wye. I can only make an assumption that the X0 terminal should be connected to the secondary windings?

Sean - Relax. Breath. No assumptions. Go look at the nameplate. Is the 208 winding Wye, or Delta? Is the 480V winding Wye, or Delta?

ice
 

SeanJo

Member
Location
Houston, TX
Sean - Relax. Breath. No assumptions. Go look at the nameplate. Is the 208 winding Wye, or Delta? Is the 480V winding Wye, or Delta?

ice

Yea thats my fault, the first response didnt post automatically so I thought it may have gotten lost in the vast interwebs and replied a second time. After calling my buddy whose job it is, he said it is in fact primary delta/secondary wye, so 480v delta windings, 208v wye windings. He did put a jumper from X0 to ground in the transformer which I've read is generally a no-go, and the creation of an ungrounded system stated earlier by Golddigger makes sense now that I look back at the setup created
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... he said it is in fact primary delta/secondary wye, so 480v delta windings, 208v wye windings. He did put a jumper from X0 to ground in the transformer which I've read is generally a no-go, and the creation of an ungrounded system stated earlier by Golddigger makes sense now that I look back at the setup created

Recommend: Don't use "primary / secondary" to refer to the windings. When you say, "Primary", that sounds like the side that is being fed - in this case, that is not the 480 winding. Suggest, "High side / Low side".

As others have noted, you are correct concerning the 208Y neutral - when using a 208Y/480D in a step up mode, leave the 208Y neutral alone. Don't tie it to anything.

As others have noted, you are correct concerning the 480D output - it is ungrounded and the line voltages to ground can float all over the place.

One last issue:
You (that is the "project you" - not necessarily "you alone") need to consider grounding the 480D or installing ground fault detection. This is a code issue. How it is done is design preference. There are only about two ways to do this (yes, there are a few other ways, but these come up the most often):
1. Corner ground the Delta winding.

2. Add a ground detector. This can be pretty simple - three light bulbs is sufficient.

ice
 

jpujols01

Member
Location
Nyc
This happened to me before, if this transformer has HO just bond it to the ground and will balance the phases in the high voltage side but don't bond the XO side of the transformer.
Again that was the solution in my case.


J Pujols.
 

Besoeker

Senior Member
Location
UK
A job I am helping on required a step-up transformer due to someone ordering the wrong equipment. The service to the building is 3phase 120/208v Y, and the new CRAC unit requires 480v 3ph. It was calculated that adding a step-up transformer would be cheaper than reordering the new CRAC unit :roll:. After wiring the transformer and starting it up, we tested voltage to be sure everything came out alright. The readings were as follows:

Primary:
A to grnd-120 A to B-208
B to grnd-120 B to C-208
C to grnd-120 A to C-208

Secondary:
A to grnd-277 A to B-480
B to grnd-277 B to C-480
C to grnd-240 A to C-480

It's also worth noting that we placed a jumper from X0 to the transformer grounding lug, even though the loads coming off of the transformer are going to be strictly 3phase Motors. Can anyone enlighten me as to why the C phase to ground voltage would be different when all coils were tapped at the same location(lug 3)? I wish I could have tried swapping 2 of the coils and seeing if it was only an issue with X3, meaning it could be an issue with the transformer itself, but it was the end of a long day and the system will not be started and operational until next week. Anyone with ideas or experience in this type of situation?

Have you checked the voltages to neutral?
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
We didn't get the output voltages we expected when. We tried the same thing.. More than 20% lower than equipment tolerance
Yeah, that is a problem with using a 480D/208Y as a step up. If the xfm has taps (most do) the taps are on the wrong winding. Still, I don't recall ever seeing an xfm over heat from this alone.

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
This happened to me before, if this transformer has HO just bond it to the ground and will balance the phases in the high voltage side but don't bond the XO side of the transformer.
Again that was the solution in my case.

Have you checked the voltages to neutral?

The xfm is 480D There is no Ho or neutral.

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
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EE (Field - as little design as possible)
From post #3

That's what I based my comment on.
Subsequent posts may have confirmed it as a Dy.

So, may we surmise you still have not read the subsequent 10 posts yet?

Bes - please consider this a gentle jerking of your chain. No intent to draw blood.

ice
 
In case all of those answers confused you, stick with the last Iceworm comment. With a non grounded delta you will get all sorts of readings. If you check the high legs to ground with a solonoid type tester such as knopps you will get nothing. For most who haven't corner grounded a delta transformer before it can be a little antimidating. You should not be bonding x0 at this point as this is would be like grounding the neutral any other place other than the service. If you corner ground it , you should have 480 volts from phase to ground on two phases and zero volts to ground on the grounded corner phase. This system is common in industrial environments. I recommend labeling the equipment as such. I have had maintenance people who only check phase to ground and think they have blown the fuse on the grounded leg, since it is reading 0 volts.
In my area we still label this as brown orange yellow, but I dont see anywhere in the code where it excludes this conductor from being labeled white or natural grey.
 
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