Xformer delta-delta with high leg situation, (irregular voltage)

[Burdaneta]

@Burdaneta We have to go one step at a time

1) You describe your supply as '120/208'. You also say 'high leg'. I am guessing that you have a high leg delta service, 240V L-L, 120V L-N on two legs, 208V L-N on the high leg. Please confirm this. The other possibility is that you have 208V L-L, 120V L-N in a balanced wye system, but I don't expect that.

2) As others have noted: when you have a delta:delta transformer wired in reverse, you do _not_ connect the neutral. Connecting the neutral when it is on the side used as the _primary_ may cause circulating currents and excess heating, but shouldn't change the output voltage.

3) You've not described how you grounded the 480V delta secondary, nor how you measured the unbalanced voltages. You should measure both L-L and L-G voltage. The voltage measurements you gave look like L-G voltages, but if the transformer secondary isn't grounded than the voltage measurements will be variable, likely not balanced, and essentially meaningless. Depending on the requirements of your system and hardware available, you might need to corner ground your 480V or get a different transformer.

-Jonathan

Hello thanks for your help Winnie.

1) indeed my supply (electrical panel ) has a high leg on phase C
2)yes is delta-delta I already disconnected the neutral thanks all for the advice on this.
3) the xformer has a self grounded on the corner with X4 I’m using that ground for everything that needs ground, like grounding bushings Etc.

I measured again I’m still have this weird voltage readings on the H side.

 

[jim dungar]

2) As others have noted: when you have a delta:delta transformer wired in reverse, you do _not_ connect the neutral. Connecting the neutral when it is on the side used as the _primary_ may cause circulating currents and excess heating, but shouldn't change the output voltage

Actually it might affect the output voltage. There can be cases where fixing the input voltage phase angles can prevent the output from adequately coping with an unbalanced loading. This is much more of a problem with primary side Wye connections

 

[infinity]

3) the xformer has a self grounded on the corner with X4 I’m using that ground for everything that needs ground, like grounding bushings Etc.

I measured again I’m still have this weird voltage readings on the H side.

Again nothing should be connected to X4. Is there something connected to it?

 

[Burdaneta]

What do you get for all 6 measurements:
A-B
B-C
C-A
A-G
B-G
C-G

-Jonathan

A-B 488V
B-C 502V
C-A 504V
A-G 275V
B-G 230V
C-G 358V

 

[infinity]

This is how it should be wired:
The 240 volts to X1-X2-X3 the output will be 480 volts on H1-H2, H2-H3, H1-H2.

Nothing connected to X4.

 

[Burdaneta]

Again nothing should be connected to X4. Is there something connected to it?

nothing connected now, but the transformer is grounded on the corner with X4 comes like that from factory

 

[infinity]

nothing connected now, but the transformer is grounded on the corner with X4 comes like that from factory

I've never seen one factory bonded can you post a photo?

 

[jim dungar]

nothing connected now, but the transformer is grounded on the corner with X4 comes like that from factory

Any factory bonding strap/connection to terminal X4 must be removed. This terminal must be floating. Among other problems you are violating the NEC by making an intentional load connection between Line and Ground.

 

[Burdaneta]

I've never seen one factory bonded can you post a photo?

https://flic.kr/p/2qwGdKU
this is the setup
You can see on the circles the ground from factory installed on the xformer

https://flic.kr/p/2qwzoqS

https://flic.kr/p/2qwFgad
ground is connected by cam-lock wire from the building ground bar

 

[LarryFine]

nothing connected now, but the transformer is grounded on the corner with X4 comes like that from factory

That's your primary now. It receives 240v delta, and does not care what the line-to-ground voltages are. It's a straight 3-wire 3ph load, like a motor. X-4 serves no purpose, and is not a neutral.

Your secondary is an isolated 3ph source. Unless you ground one wire, it's floating, and line-to-ground voltages are meaningless. You should ground one wire or install ground-detection means.

 

[Burdaneta]

That's your primary now. It receives 240v delta, and does not care what the line-to-ground voltages are. It's a straight 3-wire 3ph load, like a motor. X-4 serves no purpose, and is not a neutral.

Your secondary is an isolated 3ph source. Unless you ground one wire, it's floating, and line-to-ground voltages are meaningless. You should ground one wire or install ground-detection means.

Understand , now the question is , can use this for anything? In worried about this unusual voltages on the H side, and the purpose of this is have a 480v service inside the training area to start test ATS and some other equipments

 

[infinity]

this is the setup
You can see on the circles the ground from factory installed on the xformer

That is just a bonding jumper to the core of the transformer from the enclosure it is not factory connected to X4.

 

[Burdaneta]

That is just a bonding jumper to the core of the transformer from the enclosure it is not factory connected to X4.

Alright thanks for clarifying this point.

 

[jim dungar]

Understand , now the question is , can use this for anything? In worried about this unusual voltages on the H side, and the purpose of this is have a 480v service inside the training area to start test ATS and some other equipments

Did you really want 480V ungrounded or did you want 480/277 4wire?
The NEC requires some type of ground monitoring on ungrounded 480V. But if you corner ground your 480V side some of your ATS circuits may not like it.

 

[infinity]

Understand , now the question is , can use this for anything? In worried about this unusual voltages on the H side, and the purpose of this is have a 480v service inside the training area to start test ATS and some other equipments

What you calling unusual voltages are merely an error in reading them. The only voltage that matters is phase to phase which should be 480. Can you provide the following voltages:
X1-X2
X2-X3
X1-X3
H1-H2
H2-H3
H1-H3
Also since this is ungrounded you'll need a ground detector or you will have to corner ground the 480 side. In these scenarios it's usually best to just use a Delta/Wye step up transformer.

 

[Burdaneta]

Did you really want 480V ungrounded or did you want 480/277 4wire?
The NEC requires some type of ground monitoring on ungrounded 480V. But if you corner ground your 480V side some of your ATS circuits may not like it.

I dont need 277v , just 480v in not intended to use this service for any lights or anything that needs a neutral, just 480v for equipments required

 

[Burdaneta]

What you calling unusual voltages are merely an error in reading them. The only voltage that matters is phase to phase which should be 480. Can you provide the following voltages:
X1-X2
X2-X3
X1-X3
H1-H2
H2-H3
H1-H3
Also since this is ungrounded you'll need a ground detector or you will have to corner ground the 480 side. In these scenarios it's usually best to just use a Delta/Wye step up transformer.

That’s why in worried about

X side is ok i think
X1-X2 241V
X2-X3 250V
X1-X3 251V

H1-H2 488V
H2-H3 504V
H1-H3 502V

 

[LarryFine]

I dont need 277v , just 480v in not intended to use this service for any lights or anything that needs a neutral, just 480v for equipments required

Okay, that's fine. But, you should not leave a system completely floating like that.

Is the equipment you'll be playing with okay with a corner-grounded power source?

What is your plan for addressing the floating system? The usual options are:
1. Grounding one 480v wire.
2. Ground detection method.
3. Neutral-point derivation.

 

[infinity]

That’s why in worried about

X side is ok i think
X1-X2 241V
X2-X3 250V
X1-X3 251V

H1-H2 488V
H2-H3 504V
H1-H3 502V

Your primary voltages are a bit high which is making the secondary voltages high as well but it appears that the transformer is now wired correctly. The ungrounded issue is something to think about because that does require a remedy to make this code compliant.

 

[Burdaneta]

Your primary voltages are a bit high which is making the secondary voltages high as well but it appears that the transformer is now wired correctly. The ungrounded issue is something to think about because that does require a remedy to make this code compliant.

X2-X3 and H2-H3 as well i think is little higher because X3 is a high leg, am i wrong assuming this?, to resume you think this voltages are good to go using it on ATS , this will serve all for training purposes, we are not pretending to use this as a regular service.