Another Transformer Question

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The 3 phase primary 208 for the transformer is from a 3 phase 4 wire wye, feeding a why primary (X1, X2, X3) and the secondary is a delta 3 phase (H1, H2, H3)
 
hillbilly1 said:
Even though a neutral was not brought to the transformer on the 208 volt side, it effectively was connected because the XO bond was still connected to ground, which in turn is connected to the neutral back at the service.
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mike_kilroy said:
voltage x1-x2?
voltage x2-x3?
voltage x1-x3?
voltage x1 to 4rth wire (ground/nuetral)?
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I thought it was pretty clear in the OP. 208/120 back fed transformer supplied with L1,L2, and L3 of a 208/120 system. XO happened to be bonded. Even though a neutral was not run the equipment grounding conductor and every other conductive path connected to it carried neutral current.

Simple solution is that the XO needed to be un-bonded and left to float.

Many people have posted this scenario in the past - most of them usually destroyed the transformer. They always end up being told to never connect anything to XO, as undesired current will flow if you do use XO.
 
Telling the input power source and voltages should help explain the X0 problem; if indeed the power source is 208v grounded wye then there is no reason for the X0 on this transformer to burn up, so there is something else going on that has not been identified yet - with incompatibility of the power source. another voltage reading that could be telling is from xfmr X0 to the power source 4rth wire.....seems like there must be voltage here. Most burn up X0 issues I've seen are from differences in ground point on xfmr and power source; most common being center tapped phase on 240 for 120v loads being ground and then tied to X0 - it 120v vs 138v and heavy current flows and burns something.
 
kwired said:
I thought it was pretty clear in the OP. 208/120 back fed transformer supplied with L1,L2, and L3 of a 208/120 system. XO happened to be bonded. Even though a neutral was not run the equipment grounding conductor and every other conductive path connected to it carried neutral current.

Simple solution is that the XO needed to be un-bonded and left to float.

Many people have posted this scenario in the past - most of them usually destroyed the transformer. They always end up being told to never connect anything to XO, as undesired current will flow if you do use XO.
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I think the op's response was he knew it was wrong, he was just looking for the technical explanation on why the current was flowing.:)
 
mike_kilroy said:
Telling the input power source and voltages should help explain the X0 problem; if indeed the power source is 208v grounded wye then there is no reason for the X0 on this transformer to burn up...
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Yes there is a compelling reason for it to burn up, as I have outlined above.

I have never seen a transformer manufacturer that did not advise against connections to the neutral point terminal when a transformer is wired in a wye-delta configuration. If you do not agree with the manufacturers that there is a reason to avoid this connection, then supplying you with the detailed math that explains current distribution during unbalanced conditions (symmetrical components and such) is probably not going to change your mind.
 
Question

Question

In the example for this thread (208Y/120 is the X side of the transformer and is also the input side), let's say the XO is not connected and the bonding jumper is removed (so it is completely isolated/floating).

What voltage would you read from the XO to any of the other X terminals?
 
hardworkingstiff said:
In the example for this thread (208Y/120 is the X side of the transformer and is also the input side), let's say the XO is not connected and the bonding jumper is removed (so it is completely isolated/floating).

What voltage would you read from the XO to any of the other X terminals?
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I would say it will depend on loading conditions. If there is a heavy load on one coil it is going to draw current from both the other phases to provide what is needed to supply the load, but we have that vector math involved in determining exactly how much comes from where, if the XO is connected to the neutral of the supply and one coil is heavily loaded we pretty much have a two wire circuit involved there.

Think about motors we have a similar situation there. Many of them are Wye connected - we don't connect the source neutral to the center of the wye connected windings - we make the motor balance the load as best as it can through all three windings. If we were to conect neutral to these motors we would have this kind of discussion a lot but would be talking motors instead of transformers.
 
hardworkingstiff said:
In the example for this thread (208Y/120 is the X side of the transformer and is also the input side), let's say the XO is not connected and the bonding jumper is removed (so it is completely isolated/floating).

What voltage would you read from the XO to any of the other X terminals?
Click to expand...
With a perfectly balanced load X0-X? = 120V. An unbalanced load may cause the X0 point to move just like the neutral point on a 120/240V circuit where the source neutral conductor has been 'opened'.


Just to be pickey.:D
I know we say X-oh, but the correct terminology should be X-zero. The first character is a letter, the second is a number.
 
IEEE C57.12.00

4.1.8 Step-down operation - Unless otherwise specified, transformers shall be designed for step-down operation.

IEEE C57.12.70

2.1 General
The windings of a transformer shall be distinguished from one another as follows: two-winding transformers
shall have their windings designated as high voltage (HV or H) and low voltage (LV or X). Transformers with
more than two windings shall have their windings designated as H, X, Y, or Z.


IEEE C57.12.80

3.343 primary circuit of a regulating transformer: The circuit on the input side of the regulator.

3.346 primary winding: The winding on the energy input side. (This would be the H or HV winding in accordance with C57.12.00)

3.405 secondary winding: The winding on the energy output side. (This would be the X or LV in accordance with C57.12.00)



Based on the IEEE definitions - The transformer in question is a step-down transformer with the primary winding the H (or HV) designated terminals connected to the secondary voltage and the secondary is the L (or LV) designated terminals connected to the primary circuit.

It was commented that these terms are not defined, but I wanted to show they do have designations in the standards. The confusion in using primary and secondary is not uncommon, it is why I prefer the more universal acceptance adopted by IEC and simply state HV or LV windings and not use terms primary and secondary.
 
kingpb said:
... The confusion in using primary and secondary is not uncommon, it is why I prefer the more universal acceptance adopted by IEC and simply state HV or LV windings and not use terms primary and secondary.
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That won't work for users of the NEC unless you get 450.3 re-written.
 
don_resqcapt19 said:
That won't work for users of the NEC unless you get 450.3 re-written.
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450.3 certainly is a potential issue for confusion.

Although it would seem to me that the "primary" protection is related to IEEE C57.12.80, 3.343 - which is unrelated to the primary or secondary winding designation, but relates specifically to the input and output circuit.

But a good point none the less.
 
kingpb said:
It was commented that these terms are not defined, but I wanted to show they do have designations in the standards. The confusion in using primary and secondary is not uncommon, it is why I prefer the more universal acceptance adopted by IEC and simply state HV or LV windings and not use terms primary and secondary.
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Describing HV and LV, or H and X, does not mean directly indicate which is connected to the source.

The C57.12.00 spec you quoted says "unless otherwise specified". Almost every major manufacturer of medium sized (ie. >25kVA and <5000kVA, max 600V) power transformers uses the same general construction for both step-down and step-up units. Easily the majority of these manufacturers provide application assistance for the 'reverse-feed' of their units (i.e. http://www.acmepowerdist.com/pdf/Page_6-16.pdf). Yes, backfeeding is not always an ideal application, but it is also not always a bad one either. Evidently UL has changed their labeling requirements, I see transformers being shipped with HV and LV designations instead of the older 'primary and secondary' ones.

Generally, I prefer to follow C57.12.80 sections 3.46 and 3.405 based on not knowing the exact design specifications of the unit in a general discussion forum like this.
 
Smart $ said:
Just to be pickey.:D

I know many people besides yourself use "i.e." when presenting an example, but the more "proper" abbreviation "e.g." should be used.
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Thank you.:thumbsup:
You made me go look it up. You are correct, effectively i.e. is used instead of "in other words" and e.g. is used instead of "for example".
 
jim dungar said:
Thank you.:thumbsup:
You made me go look it up. You are correct, effectively i.e. is used instead of "in other words" and e.g. is used instead of "for example".
Click to expand...

And That is why MH is tops I learned something I have been doing wrong for eons....

Thanks.
 
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