GEC required for 208-480 delta transformer?

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kwired said:
me too.




If the low voltage were the intended primary side, wouldn't they ordinarily be marked as "H1, H2, H3, H0?

I agree that it may be designed to be reverse fed.
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I guess you could reverse feed it with 480v to get 208v Y secondary but the part number is a configuration for a 208v Y Primary 480v Delta secondary.
The "28" after the V is representative of the 208v Y Primary.
A "48" after the V would be representative of a 480v Primary.

That's what was confusing to me about this all along.


JAP>
 
kwired said:
me too.
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The Link in Post #31 resolution #3 is exactly what made the smoke fly in the back fed Xfmr I was talking about earlier.

It was an existing standard 480v Dela 120/208v Y Secondary Transformer they had where they failed to remove the bond from XO to the case when they back fed the 208v Secondary side to try and get 480 out of the primary.

There was a 400 amp 208v fused disconnect ahead of it.
As soon as they threw the handle up to energize it, the result reared it's ugly face.

JAP>
 
I don't know the technical reason why but I think it was because the transformer windings that were back fed were "Cut Short" by the Neutral bond to ground making the winding an actual load that didn't have enough resistance to handle the voltage that was applied to it.

Much like the result of sending 208 or 277v to a 120v ballast but at a much larger scale.


JAP>
 
If a married couple is introduced as Mr. and Mrs. Smith I'd like to think that most would consider the "Primary" or first name that was used was referring to the Male just as the first number after the V is 28 which stands for 208v Y.


JAP>
 
jap said:
A Transformer is a dumb animal, it doesn't really care, but the part number is definitely for a 208v Y primary 480v Delta secondary.


JAP>
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I agree transformers are dumb, and at one time you could hook them up either way and nobody cared. Then the busy body, hand wringing types did what they always do and got in everybody's business and said the transformer had to be allowed by the manufacturer to be reverse fed. I'll bet that's why the part number comes up like it does. I'll bet it's the same transformer as the step down version.

I've hooked up factory made step up transformers because my boss insisted on using them. They don't have an X0 on the low side. I maintain that the XO on this transformer should be left floating.
 
Simplified truths about transformers, that have served me well.

H and X do not refer to primary and secondary. Industry standards are for the High Voltage windings to use "H" as their terminal designation, and for Low Voltage windings to use "X".

The winding configurations is what it is. Winding configurations of Delta and Wye should not be used to describe the number of wires used in a circuit.

Do not 'run a wire to' the neutral point on the input (primary side) of a wye connected transformer, unless you are the utility.
 
jim dungar said:
Simplified truths about transformers, that have served me well.

H and X do not refer to primary and secondary. Industry standards are for the High Voltage windings to use "H" as their terminal designation, and for Low Voltage windings to use "X".

The winding configurations is what it is. Winding configurations of Delta and Wye should not be used to describe the number of wires used in a circuit.

Do not 'run a wire to' the neutral point on the input (primary side) of a wye connected transformer, unless you are the utility.
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:thumbsup:
 
:thumbsup:
jim dungar said:
Simplified truths about transformers, that have served me well.

H and X do not refer to primary and secondary. Industry standards are for the High Voltage windings to use "H" as their terminal designation, and for Low Voltage windings to use "X".

The winding configurations is what it is. Winding configurations of Delta and Wye should not be used to describe the number of wires used in a circuit.

Do not 'run a wire to' the neutral point on the input (primary side) of a wye connected transformer, unless you are the utility.
Click to expand...

:thumbsup: x2

I've never ran a neutral wire to the input side of a transformer of course unless it called for 120v 1ph primary.


JAP>
 
kwired said:
...
If the low voltage were the intended primary side, wouldn't they ordinarily be marked as "H1, H2, H3, H0?
...
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No. "H" designations always indicates the higher voltage side.

Not certain why "X" designations are used on the lower voltage side. Many assume it to designate secondary connections, and could well be the orignal concept for using "X" designation. However, "X" designations only indicate secondary connections on step-down transformers.
 
This is not the first time this has come up here, though I don't recall exactly why, but a single core transformer like the dry types most of us use for under 600 volts has more problem with connecting the wye point of a wye primary to the supply neutral, then a bank of transformers where you have multiple cores - like what is typical for POCO pole top transformer banks.

Most people that have posted here with problems and we find out they connected the neutral on a wye primary are not burning the thing out in just seconds but they definitely are usually having high amounts of current flowing and causing problems, I still think when Jap said he had one go out really quickly there was more to it then just the supply neutral being connected, maybe there was another fault somewhere that nobody ever noticed?
 
jim dungar said:
Briefly
The connection of the X0 fixes the center point of the wye which affects its response to unbalanced loads on the secondary.
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More importantly, I think, is that it behaves very badly in the presence of unbalanced line to neutral voltages on the primary (which may or may not be caused by VD and unbalanced secondary loads.)
 
jim dungar said:
Briefly
The connection of the X0 fixes the center point of the wye which affects its response to unbalanced loads on the secondary.
Click to expand...

GoldDigger said:
More importantly, I think, is that it behaves very badly in the presence of unbalanced line to neutral voltages on the primary (which may or may not be caused by VD and unbalanced secondary loads.)
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Gee.... when I said brief, I was hoping for something a little more informative than what I could surmise. :slaphead:

If you do not care to elaborate just a little bit more, perhaps provide a link to some documentation.
 
Smart $ said:
Gee.... when I said brief, I was hoping for something a little more informative than what I could surmise. :slaphead:

If you do not care to elaborate just a little bit more, perhaps provide a link to some documentation.
Click to expand...

OK. This has been discussed in other threads, but I can't find a good link right now.

Look at it this way:

In a wye-delta transformer each L-L voltage in the delta corresponds, with a factor of the turns ratio, to a single L-N voltage in the wye. Lets call the wye voltage that corresponds to VAB on the delta side Van.



Because the three L-L voltage vectors (phasors) form a closed triangle their sum must obey a simple vector equation: VAB +VBC + VCA = 0 (where a bold character represents a vector.)

That means that on the wye side Van + Vbn + Vcn must also = 0.

Since the actual wye side voltages are somewhat independent (taps, primary imbalance, IR drops from unbalanced load, etc.) this equation will NOT generally be satisfied. The result is that no-load current will flow in the delta windings and wye windings until the individual voltage drops allow both equations to be satisfied. Depending on the magnitude of the voltage imbalance this may result in neutral current which is far greater than the design full load current of the transformer. If large enough it will trip the OCPD. If smaller, it will just overheat the transformer or the unprotected neutral wire.

If you leave the star point unconnected it will simply float (with no need for current) to a point that allows the second equation to be satisfied. And there will be no neutral current regardless of how balanced or unbalanced the delta load may be.

If that does not get you thinking on the right track, let me know and I will try to answer more questions. :)
 
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