480 Delta or maybe not?

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maloo

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Marion, Iowa
I saw a case were the utility set a 480 Y transformer, ran all phases and neutral to a CT cabinet but only the the phase wires were from to the service disconnect which is a 2000 amp MCC that was setup as a delta. All the loads are three phase motor loads except for a transformer. Is this an acceptable practice? I would think that the utilities transformer should be a delta transformer. Also is it required by the NEC to us ground alarms when using a ungrounded delta? I have not found a reference.
 

physis

Senior Member
I'm no three phase expert, but if you leave out the center of a wye you have a delta. I can't think of a reason, not that the code book couldn't come up something, to not treat a wye with the center omitted as a delta, cause that's what it is.

Also is it required by the NEC to us ground alarms when using a ungrounded delta?

Don't you have to ground a delta too?
 

physis

Senior Member
I saw a case were the utility set a 480 Y transformer, ran all phases and neutral to a CT cabinet but only the the phase wires were from to the service disconnect which is a 2000 amp MCC that was setup as a delta. All the loads are three phase motor loads except for a transformer. Is this an acceptable practice? I would think that the utilities transformer should be a delta transformer. Also is it required by the NEC to us ground alarms when using a ungrounded delta? I have not found a reference.

Wait a second, are you describing a situation where a wye tranformer with the center grounded is feeding a near by panel as delta and that's going to also have to be grounded but from a hot leg?
 

augie47

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On a couple of rare occasions I have had POCO set a wye pad (all they carry) and the customer had wired for and requested delta. In those cases, POCO disconnected the bond strap on XO in the transformer and only terminated the phases. I might have been in error, but I saw no problem.
If the bond is still in place or they had made an XO/earth connectiomn of any type, I would think ther would be a problem.
Just an opinion.
 

jim dungar

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I'm no three phase expert, but if you leave out the center of a wye you have a delta.
No.

A wye and a delta are apples and oranges. Wye and delta describe the physical interconnection of the transformer windings. The generic terms wye and delta say nothing about the number of conductors in the system nor how it is grounded.

What the OP is describing is a 4-wire wye transformer feeding a 3-wire piece of equipment.

If the neutral point of the wye transformer is solidly grounded, then it must be brought to the service equipment even though the load does not require it. It does not need to go any further than the service entrance (the equipment does not need a neutral bus). And at 2000A ground fault protection would be required as the voltage to ground is more than 150V.
 

physis

Senior Member
No.

A wye and a delta are apples and oranges. Wye and delta describe the physical interconnection of the transformer windings. The generic terms wye and delta say nothing about the number of conductors in the system nor how it is grounded.

What the OP is describing is a 4-wire wye transformer feeding a 3-wire piece of equipment.

If the neutral point of the wye transformer is solidly grounded, then it must be brought to the service equipment even though the load does not require it. It does not need to go any further than the service entrance (the equipment does not need a neutral bus). And at 2000A ground fault protection would be required as the voltage to ground is more than 150V.

I don't understand the "no".

We pretty much agree on everything except that if you ignore the center of a wye you have a delta.
 

charlie b

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We pretty much agree on everything except that if you ignore the center of a wye you have a delta.
In this point, I agree with Jim. But the equipment being served will agree with you. The physical difference between a WYE and a Delta has to do with the relationships between "phase current" (i.e., phase meaning the internal winding of the transformer) and "line current" (i.e., line meaning the wires between the transformer and the loads), as well as the relationship between "phase to ground voltage" and "phase to phase voltage." Those relationships are different for a Delta and for a WYE with a disconnected neutral. However, once you draw a box around the transformer, and promise not to look inside that box, and are therefore viewing the system from the only perspective that is available to the load, you will not be able to distinguish a Delta source from a WYE source with a disconnected neutral.

It is very much analogous to the difference between a gamma ray and an X-ray (I am harking back to my days in the Nuclear Navy). One comes from a decay internal to the nucleus of an atom, and the other comes from a decay in the orbit of an electron surrounding the nucleus of an atom. Very different, with very different impacts on the atom itself. But if all you saw was one of these things going past you, without being able to see where it came from, you won?t be able to tell whether it was a gamma or an X-ray. From your perspective, they are identical.
 

physis

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If you don't use a wye's center although it's coils in the transformer are still there they have no effect on the transformer's behaviour as a delta.
 

charlie b

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If you don't use a wye's center although it's coils in the transformer are still there they have no effect on the transformer's behaviour as a delta.
The classic non-argument argument:
Person One: I contend that the grass is GREEN.
Person Two : No, you are wrong. The sky is BLUE.
Let me say again that,
(1) The grass is green, and
(2) A Delta and a WYE with a disconnected neutral are
- - - - - (a) Very different, internal to the transformer, and
- - - - - (b) Not distinguishable, external to the transformer, and finally,
(3) It is therefore not correct to say that once you disconnect the neutral of a WYE, the device becomes a Delta.
 

physis

Senior Member

The classic non-argument argument:
Let me say again that,
(1) The grass is green, and
(2) A Delta and a WYE with a disconnected neutral are
- - - - - (a) Very different, internal to the transformer, and
- - - - - (b) Not distinguishable, external to the transformer, and finally,
(3) It is therefore not correct to say that once you disconnect the neutral of a WYE, the device becomes a Delta.

I don't even know how to argue with that Charlie. :grin:

Edit: Except that if you put the tranformer in a black box your item three is wrong.
 
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charlie b

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Edit: Except that if you put the tranformer in a black box your item three is wrong.
Not at all. My item three talks about the device. If the transformer is in a black box, that fact does not alter the nature of the transformer itself.:wink:

It's like the old saying: If a tree falls in a forest, and nobody is there to observe the event, is the man still wrong and is his wife still right? :D
 

winnie

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Springfield, MA, USA
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Similar to the myriad uses of the term 'phase', the terms 'wye' and 'delta' have several uses in the industry. Look at the original post, where the MCC is described as 'setup as a delta'.

As used to describe transformers, wye means one sort of winding, delta means another sort of winding. 'Zig-zag', 'Scott T', etc. describe yet more esoteric windings.

Wye and delta are also used to describe _services_; a wye service is a three phase service that has a neutral point that is at the 'center' of the three 'phase' voltages. A delta service does not have this neutral point; it may have a 'center tap' on one of the side legs.

Suitably connected, a transformer with a wye secondary can provide a delta service. So could a transformer with a zig-zag secondary or a T secondary.

Suitably connected, a transformer with a wye secondary can provide a wye service. So could a transformer with a zig-zag secondary or a T secondary or even a transformer with a delta secondary in combination with a separate zig-zag transformer.

I agree with Charlie B that the description of the inner workings (which might require a sawzall to determine) is what it is; a wye secondary doesn't suddenly become a delta secondary just because of its external connection. However a wye secondary can supply a delta service.

Back to the original post: this 'neutral' conductor that was brought to the CT cabinet, was it grounded at the transformer or CT?

-Jon
 

physis

Senior Member

Not at all. My item three talks about the device. If the transformer is in a black box, that fact does not alter the nature of the transformer itself.:wink:

It's like the old saying: If a tree falls in a forest, and nobody is there to observe the event, is the man still wrong and is his wife still right? :D

OK Charlie :grin: :grin:

How have you been anyway? It's been quite a while since I've been able to hang out with you guys and I've missed that.
 

charlie b

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How have you been anyway? It's been quite a while since I've been able to hang out with you guys and I've missed that.
And we've missed you as well, Sam. I've been meaning to say that it's nice to have you back again.

I've been fine. I'm working for a company that is doing very well (i.e., staying busy). These days, that is as much to hope for as is safe to, well, to hope for. :wink:

(OT: I haven't gotten out with my camera as much as I could like. But I do have some nice new work in my "favorites" collection, if anyone is interested: http://www.pbase.com/cbeck/favorites&page=all )
 

physis

Senior Member
I agree with Charlie B that the description of the inner workings (which might require a sawzall to determine) is what it is

Agreed.

a wye secondary doesn't suddenly become a delta secondary just because of its external connection.

Agreed.

I never said the transformer changes. Only that you can not just emulate but have delta output, from a wye transformer.

It's rather interesting to me how much impedance (it's different than resistance :grin:) to this there's been.

Has anybody ever used a center tapped transformer for something and not needed the center tap. This is no different.
 
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physis

Senior Member

And we've missed you as well, Sam. I've been meaning to say that it's nice to have you back again.

I've been fine. I'm working for a company that is doing very well (i.e., staying busy). These days, that is as much to hope for as is safe to, well, to hope for. :wink:

(OT: I haven't gotten out with my camera as much as I could like. But I do have some nice new work in my "favorites" collection, if anyone is interested: http://www.pbase.com/cbeck/favorites&page=all )


Just like the old days Charlie, posting a minute before I was able to land my last post. :grin:

It is nice to be back here. But the place has changed a bit. Everybody's a moderator anymore, and I have to start out as a nobody again. But it's good that most of the people I've enjoyed agreeing and debating with are still here, aside from a couple I haven't seen. And it's also really good to see Carlie Eldridge back again.
 

jim dungar

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A wye transformer without the center wire is a 3-wire source it is not a delta source. A delta connection with a center tap is a 4-wire source it is not a wye source.

It is absolutely wrong to say a piece of equipment is set up as a delta just because it only needs 3-phase 3-wire.For example, most new generation VFD's only need 3-wires (no neutral), but they need to be fed from a wye system where the center point is grounded so that the L-G voltage is 277V. If you feed these with a corner-grounded delta system, internal components will fail.
 

pfalcon

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Location
Indiana
At our industrial site the old line sources are from ungrounded delta, the new are from high-impedance wye. The buss only carries the ungrounded phases so that if a single phase-ground fault occurs the plant still runs. The building steel is interconnected and bonded to earth to provide a protective ground. Each machine is then bonded to the building steel.
 

jim dungar

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One difference is how they behave if the system is bonded to ground.
Another difference is how zero sequence currents are handled.

As far as wiring of equipment goes, 3-phase 3-wire is almost never dependent on whether the source is a delta or a wye.
 
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