Transformer Question

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AmeL

Member
Location
STL, MO
No paper trail on this transformer but i was told that it is a spare for the current 1500kVA 4160/480V delta delta corner grounded secondary system that we have. Anybody know what do the switch positions on the picture #2 represent. Notice barely readable nameplate calling this 480Y/277 but CX1, BX2 and AX3 labeled blue white and red as the standard labeling we use for the current system.
As always your time and input is very appreciated.
Sincerely
Amel
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
No paper trail on this transformer but i was told that it is a spare for the current 1500kVA 4160/480V delta delta corner grounded secondary system that we have. Anybody know what do the switch positions on the picture #2 represent. Notice barely readable nameplate calling this 480Y/277 but CX1, BX2 and AX3 labeled blue white and red as the standard labeling we use for the current system.
As always your time and input is very appreciated.
Sincerely
Amel
edit to add: I'm a slow poster. So as Larry said:
Switch positions are likely a tap changer. The nameplate shows different primary voltages that likely correspond to the tap changer positions.

It's a 480/277Y secondary. I suspect it has been hooked up ignoring the neutral and corner grounding one of the phases (BX2 probably)

ice
 

AmeL

Member
Location
STL, MO
Thanks very much. The numbers do represent what Larry said.
Ice, how is that delta connection done (inside transformer I presume) and can it be rewired back to Y?
Thanks
 

Smart $

Esteemed Member
Location
Ohio
... how is that delta connection done (inside transformer I presume) and can it be rewired back to Y?
Thanks
The xfmr secondary is wye configured. There is nothing to rewire. If you connect to a 3-wire, corner-grounded system, you simply ignore the X0 terminal... and your output will be for the most part no different than a 480 delta-configured secondary.

I believe the assumption was made that since one of your line colors is white, your current system is corner grounded, as it is a violation to use white for ungrounded conductors.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Thanks very much. The numbers do represent what Larry said.
Ice, how is that delta connection done (inside transformer I presume) and can it be rewired back to Y?
Thanks
As S$ said the transformer looks like it was previously connected with b phase as the corner ground. But, I would not count on that being true.

As you said, this xfm has been laying around long enough that no one knows the history. So naturally, before being pressed into service, you would test it to make sure it had not died from lack of use laying in the corner.

Rarely, but occasionally, the XO terminal is internally connected to the transformer case. So naturally, one of the tests you would do is to make sure the secondary windings were not grounded (to the case). This will also tell you the XO is not internally bonded.

And if I were going to connect the transformer as a wye secondary, then the Secondary winding resistance test - line to XO - tells you that some devious nut didn't go in and internally disconnect the XO. It's not likely, but since you don't know the history, I'd check.

ice
 
As S$ said the transformer looks like it was previously connected with b phase as the corner ground. But, I would not count on that being true.

As you said, this xfm has been laying around long enough that no one knows the history. So naturally, before being pressed into service, you would test it to make sure it had not died from lack of use laying in the corner.

Rarely, but occasionally, the XO terminal is internally connected to the transformer case. So naturally, one of the tests you would do is to make sure the secondary windings were not grounded (to the case). This will also tell you the XO is not internally bonded.

And if I were going to connect the transformer as a wye secondary, then the Secondary winding resistance test - line to XO - tells you that some devious nut didn't go in and internally disconnect the XO. It's not likely, but since you don't know the history, I'd check.

ice
I think one of the problem you may encounter is that if you connect this to the existing system and it is a Service Entrance equipment, the corner grounding MAY also have been done there. So make SURE that

  • none of the downstream phases are grounded, and if so hunt them down and destroy them,:D
  • you establish the grounding of the Xo at the transformer,
  • have them make up their minds about the labeling as X1 should be labeled as A phase, and X3 as phase C,:D
  • make sure that the rotation is not reversed on the downstream equipment.
 

AmeL

Member
Location
STL, MO
Just checked and the Xo is not internally disconnected and not connected to the case. But the transformer that is currently in use is the same type and has same markings. ( see attached pic) If nothing is done internally to change the configuration and no phases are grounded downstream and from what it looks like they just grounded center or B leg..... what is that system called? Is it a 3 phase or 2 phase? How does this change the rating of it?

Also I wonder how the calculations work ... and where can I get references on how to calculate this ... What does a phasor diagram look like? Power quality analyzer would be nice now.
Thanks again for all your inputs.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If nothing is done internally to change the configuration and no phases are grounded downstream and from what it looks like they just grounded center or B leg..... what is that system called? Is it a 3 phase or 2 phase? How does this change the rating of it?
Corner-grounded Delta.

3 phase. Line-to-line loads don't care which, if any line is grounded.

It doesn't.

Also I wonder how the calculations work ...
See second answer above.

Wait for the engineers among us for the rest.
 
Just checked and the Xo is not internally disconnected and not connected to the case. But the transformer that is currently in use is the same type and has same markings. ( see attached pic) If nothing is done internally to change the configuration and no phases are grounded downstream and from what it looks like they just grounded center or B leg..... what is that system called? Is it a 3 phase or 2 phase? How does this change the rating of it?

Also I wonder how the calculations work ... and where can I get references on how to calculate this ... What does a phasor diagram look like? Power quality analyzer would be nice now.
Thanks again for all your inputs.
It sound like you have a common Delta/Wye transformer and the have chosen to isolate the Xo, instead of grounding it and grounded one of the Y legs. Never heard or see it done this way but from the theoretical standpoint it can be done. I would have an engineer review the system and convert it to a properly grounded Xo.
 
Corner-grounded Delta.

3 phase. Line-to-line loads don't care which, if any line is grounded.

It doesn't.

See second answer above.

Wait for the engineers among us for the rest.
I hear what people saying but the only evidence - the picture - tells me that it is a standard D/Y trafo. On the picture I don't see ANY extra wire on the center phase, each leg seems to have 3 wires landing on them and that would mean parallel feeders to me.
 

Smart $

Esteemed Member
Location
Ohio
I hear what people saying but the only evidence - the picture - tells me that it is a standard D/Y trafo. On the picture I don't see ANY extra wire on the center phase, each leg seems to have 3 wires landing on them and that would mean parallel feeders to me.
Look more closely... there are four wires on the X2 terminal.

Even if there were only three, it could be grounded elsewhere.
 

AmeL

Member
Location
STL, MO
Yes there is a fourth bare copper wire tied to the X2 terminal.
It sound like you have a common Delta/Wye transformer and the have chosen to isolate the Xo, instead of grounding it and grounded one of the Y legs. Never heard or see it done this way but from the theoretical standpoint it can be done.
I think that this is the case but what benefits are there in doing this?


Larry just out of curiosity why would this be called a 3 phase delta?
Thanks
 
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Smart $

Esteemed Member
Location
Ohio
Yes there is a fourth bare copper wire tied to the X2 terminal.
I think that this is the case but what benefits are there in doing this?
One benefit is that you have a grounded 3? 3-wire system. Downstream you only need two-pole breakers for 3? loads. Less wire, less equipment equals less cost


Larry just out of curiosity why would this be called a 3 phase delta?
(Not Larry, but...) Some people call it a delta simply because it is a 3? 3-wire system... and even though a wye secondary is supplying the power, you can only connect line-to-line, as with a 3-wire delta secondary.
 
Look more closely... there are four wires on the X2 terminal.

Even if there were only three, it could be grounded elsewhere.
I can't say that I see the 4th wire distinctly on the X2, but I understand why one can believe that is what they see.

If that is the case than we have a why 480V connection where the Xo is insulated and isolated and the X2 is grounded. I do NOT believe that the windings were re-connected to Delta, because if a well equipped shop that is capable and competent to perform such modification they would have ALSO changed the nameplate. If they kept the existing wiring then it would be 277V Delta and they would had to rewind the secondary completely if it to provide 480V Delta. If they performed any of the above, then it is safe to assume that they would have changed the nameplate also.

I have also commented in my other post that the system - which the transformer is connected - too should be checked for grounding.

So my recommendation would be to remove the insulation from the Xo, connect it to the ground and remove ALL other grounding connections from ANY of the phase wires. (Of course this presumes that all primary and secondary voltages are tested and found to be in compliance with the nameplate data.)
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Plus, check or provide overcurrent protection for the previously-grounded conductor where applicable.
 

Smart $

Esteemed Member
Location
Ohio
I can't say that I see the 4th wire distinctly on the X2, but I understand why one can believe that is what they see.
If you just click on the thumbnail, the picture opens sized slightly smaller than your browser window will permit. If you open in new window, it'll open at screen resolution. Actual resolution is 1720 x 1029... but quite grainy. However, I can "make out" four wires, three of which are phase taped (white?) and bundled with a white cable tie. I can also "make out" four mechanical lugs cxonnected to the terminal busbar of X2.
 

Smart $

Esteemed Member
Location
Ohio
...

So my recommendation would be to remove the insulation from the Xo, connect it to the ground and remove ALL other grounding connections from ANY of the phase wires. (Of course this presumes that all primary and secondary voltages are tested and found to be in compliance with the nameplate data.)
Plus, check or provide overcurrent protection for the previously-grounded conductor where applicable.
Not a whole lot to check but if corner grounded as suspected, that may be a lot to change when it isn't necessary...:confused:
 
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