480V Primary 208V Secondary Transformer question

[iwire]

Never say never. If it's a three phase autotransformer the neutral passes through and is common to both sides.

It's a delta wye, there is no neutral that passes through.

 

[iwire]

I doubt the burned Xo, the noise, with a missing input phase has ANY THING to do with tying Xo to the 480 side:

I disagree, I have had co-workers have the same issue for the same reason.

Not only did the OP state the XO is tied to the load it is also grounded to the frame.

You could not burn up XO with out it being connected to ground.

 

[mike_kilroy]

I'm from Missouri; show me how it can be so.

I'm from Missouri; show me how it can be so.

I doubt the burned Xo, the noise, with a missing input phase has ANY THING to do with tying Xo to the 480 side: I bet these issues were caused by 2 phases on primary trying to power 3 balanced phases of secondary load - that required a real Xo.

I disagree, I have had co-workers have the same issue for the same reason. Not only did the OP state the XO is tied to the load it is also grounded to the frame. You could not burn up XO with out it being connected to ground.

So we agree and disagree? OK, so I took the time to draw up the wiring diagram to prove that there is no current flow BACK from the 480v delta winding to the primary Xo transformer terminal. You say current is flowing OUT from the 480v isolated secondary coils, thru the Xo back to the primary Xo terminal on the transformer. Please show the RETURN path for this phantom current coming BACK into the 480v secondary coils?

In addition, the Xo path IS GROUNDED BETWEEN THE TRANSFORMER AND THE Chiller/MRI loads; So even if somehow there is path for the outgoing current down the Xo from the secondary, it will go to ground - NOT the primary Xo - where the OP said the terminal damage was located, no?

The only suggestion that makes any sense to me that current flowed out of (or into) the Xo terminal on the transformer causing heating is if it came out of the transformer Xo itself on way to that ground. I have stuff to do so will not take time to figure out the physics of loosing a primary phase on loaded transformer, and how that imbalance to a balanced delta secondary would effect the Xo currents. I imagine it would, and THAT is the reason for the Xo overheating at its terminal.

 

[kwired]

If you wye connect 277 volt loads to a 480 source (wye or delta) and leave the mid point of the wye float, those loads work normally if they are all the same resistance. When the load is unbalanced you will have a shift in voltage dependent on the unbalance of the load resistances. Connecting that mid point to ground (but not to the source coils) which is what happened when OP's install connected those "neutrals to the primary side "XO", won't make any difference, all it does is establish a ground reference, that still varies in voltage to other points in the system with changing load conditions. If the secondary were a grounded wye, you just may have objectionable current flowing on non current carrying bonded parts, but since the secondary is delta and ungrounded the "secondary neutral" is a "floating neutral".

 

[GoldDigger]

This keeps coming back up every time someone grounds (or connects to neutral) the primary X/H0 terminal.
The delta winding forces the voltages (phasors) AB, BC, and CA to form a closed triangle.
That imposes the exact same equality condition on AN + BN + CN.
During a phase loss that equation cannot be satisfied on the primary side and enormous currents will flow in the 0 terminal trying to make the voltages equalize.
It can even cause large problems without a phase loss if the supply voltages are even slightly unbalanced for any reason.

Sent from my XT1585 using Tapatalk

 

[ksmith846]

This keeps coming back up every time someone grounds (or connects to neutral) the primary X/H0 terminal.
The delta winding forces the voltages (phasors) AB, BC, and CA to form a closed triangle.
That imposes the exact same equality condition on AN + BN + CN.
During a phase loss that equation cannot be satisfied on the primary side and enormous currents will flow in the 0 terminal trying to make the voltages equalize.
It can even cause large problems without a phase loss if the supply voltages are even slightly unbalanced for any reason.

Sent from my XT1585 using Tapatalk

Exactly what happens here......line in is 208 feeding Wye secondary side......the 480 Delta side feeds a 480 volt panel which feeds two 3 phase 20 amp chiller loads and one 3 phase 4 wire MRI thru a disconnect and control panel (with the 480 V panel neutral being connected the low side X0).......this Neutral is being used inside of the MRI control Cabinet, so this is the only Neutral load under normal condition.......this has no major imbalance. Which the 100 amp EQG can handle without showing any problem....SO everything works, although wrong.

Once FPL loses any phase, the unbalanced load created by the lost phase is trying to get back to its source......which is the Neutral wire from the 480 volt panel which is lost because it's connected to the Low side XO. The current cannot get anywhere so it goes thru the Equipment Ground run with the 208V feeder.........which is terminated on the 208V MDP ground bar......this connection is the weakest point so this is where the massive current is over heating the insulation and burning the wire back.

I can't believe that the wrong TX was submitted and approved by the EOR,......then installed by a Licensed Journeyman and then inspected during the AHJ service inspection......it has been pure luck it has worked for 2 years without anything being destroyed.

 

[mike_kilroy]

Exactly what happens here.....

Once FPL loses any phase, the unbalanced load created by the lost phase is trying to get back to its source......which is the Neutral wire from the 480 volt panel which is lost because it's connected to the Low side XO. The current cannot get anywhere so it goes thru the Equipment Ground run with the 208V feeder.........which is terminated on the 208V MDP ground bar......this connection is the weakest point so this is where the massive current is over heating the insulation and burning the wire back.

WRONG. PLEASE reread my last post and Digger's last post. Your statement above is 100% incorrect.

I PROVED to you there is not a return path for current out of your 480v secondary into the Xo. None. Zip. Zilch. Your 480V is NOT AFFECTING ANYTHING HAPPENING ON THE Xo line from your primary. TOTALLY SEPARATE. ISOLATED.

Said another way, your Xo line into your 480v chiller/mri is NOT AFFECTING ANYTHING HAPPENING ON THE 480V side.

Digger just explained to you why your Xo terminal overheated & why your transformer became very upset and buzzed and shook at you: it is 100% due to unbalanced primary currents trying to still supply 3 secondary windings with balanced loads. Nothing at all to do with the 480v side!

I hope the replacement transformer arrives tomorrow as promised so you get a REAL Xo before someone is hurt!

 

[iwire]

WRONG. PLEASE reread my last post and Digger's last post. Your statement above is 100% incorrect.

I PROVED to you there is not a return path for current out of your 480v secondary into the Xo. None. Zip. Zilch. Your 480V is NOT AFFECTING ANYTHING HAPPENING ON THE Xo line from your primary. TOTALLY SEPARATE. ISOLATED.

Said another way, your Xo line into your 480v chiller/mri is NOT AFFECTING ANYTHING HAPPENING ON THE 480V side.

Strange that in the cases I know of the conductor bonding XO to ground melted down not the terminal.

You have proved nothing to me.

 

[GoldDigger]

Strange that in the cases I know of the conductor bonding XO to ground melted down not the terminal.

You have proved nothing to me.

We never said that there was not current from X0 to ground and from there back to the utility transformer secondary neutral.
But the reason for the current is simply the existence of the delta secondary, not the presence of any kind of loads on it.
In addition to the circulating primary current there will be similarly high currents in the delta windings themselves.
The conductor probably fails because it is undersized relative to the well heat sinked terminal and secondary conductor.

And I am not particularly concerned about proving anything to you, just stating the facts.

Sent from my XT1585 using Tapatalk

 

[iwire]

We never said that there was not current from X0 to ground and from there back to the utility transformer secondary neutral.

Mike seems to be saying exactly that.

But the reason for the current is simply the existence of the delta secondary, not the presence of any kind of loads on it.
In addition to the circulating primary current there will be similarly high currents in the delta windings themselves.
The conductor probably fails because it is undersized relative to the well heat sinked terminal and secondary conductor.

Sent from my XT1585 using Tapatalk

If XO was not connected to ground no current would flow on it.

The descriptions I got from my co-workers were that the conductor failed not the terminal, considering they saw it and we did not I am not going to argue over it

 

[iwire]

And I am not particularly concerned about proving anything to you, just stating the facts.

Sent from my XT1585 using Tapatalk

Whatever ....

 

[ksmith846]

WRONG. PLEASE reread my last post and Digger's last post. Your statement above is 100% incorrect.

I PROVED to you there is not a return path for current out of your 480v secondary into the Xo. None. Zip. Zilch. Your 480V is NOT AFFECTING ANYTHING HAPPENING ON THE Xo line from your primary. TOTALLY SEPARATE. ISOLATED.

Said another way, your Xo line into your 480v chiller/mri is NOT AFFECTING ANYTHING HAPPENING ON THE 480V side.

Digger just explained to you why your Xo terminal overheated & why your transformer became very upset and buzzed and shook at you: it is 100% due to unbalanced primary currents trying to still supply 3 secondary windings with balanced loads. Nothing at all to do with the 480v side!

I hope the replacement transformer arrives tomorrow as promised so you get a REAL Xo before someone is hurt!

Please excuse my obvious lack of explanatory ability and lower level of knowledgeable experience in comparison to your own. That's why I presented this question here in the first place. To get help with something which stumped me.

See the attached drawing for reference..........and note where the melt down is occurring when a POCO phase is lost. Where ever the imbalance is occurring, which you have so kindly informed me, is on the 208V line side.

Not at XO.....not on the frame connections.....not in the HA panel........wire run from XO thru the frame ug and terminated in Panel HA.......no where except the Line side equipment ground connection in the 208V MDP

 

[mike_kilroy]

We never said that there was not current from X0 to ground and from there back to the utility transformer secondary neutral.

Mike seems to be saying exactly that.

Sorry for apparent confusion on my post.

I did NOT say that.

I was simply saying what digger came back and explained: a major imbalance on the input to the transformer caused the Xo hi current - BACK TO THE UTILITY - NOT to the secondary!!!!

I have tried to be clear as mud: there is absolutely ZERO current flow from the 480v secondary windings to the Xo described in the OP.

I am not sure how to say this more clearly.

If I am wrong, OK - prove me wrong. But please do not misunderstand what I wrote and drew a picture of, and assign different meaning to my post..

 

[ksmith846]

Sorry for apparent confusion on my post.

I did NOT say that.

I was simply saying what digger came back and explained: a major imbalance on the input to the transformer caused the Xo hi current - BACK TO THE UTILITY - NOT to the secondary!!!!

I have tried to be clear as mud: there is absolutely ZERO current flow from the 480v secondary windings to the Xo described in the OP.

I am not sure how to say this more clearly.

If I am wrong, OK - prove me wrong. But please do not misunderstand what I wrote and drew a picture of, and assign different meaning to my post..

I understand there is no current flow from the 480 winding's back to the Utility.....I stated it wrong earlier I know....I was trying to refer that wherever the imbalance was coming from that it was making it back to my EG at the MDP and failing that wire....

The phase imbalance is coming thru the XO wire we ran to the TX frame connection and back through my 208V, line side, equipment ground where it's over heating at it's termination......

 

[ksmith846]

Here's what I do know........the submitted, reviewed, approved, supplied, installed and inspected transformer was the wrong one.

Which I, as the qualifier and owner, was appalled to discover even occurred. I am also the one whose rear and lively hood is on the line for anything that may happen. We have had 2 calls out there for a loud transformer and the reports came back as just a lost phase. I thought nothing of it at the those times.

I went to job myself this last time, read the nameplate and realized it was the wrong type. Knowing that it had to be replaced right away.

I only came on here to get input as to how it could still be able to work. I had also read on the internet that a delta wye could be wired in reverse and somehow still get a neutral off of the delta side somehow. I did not believe that either so I posted here to get advice.

I am the one buying and paying to have a new one installed. I am the one who hasn't slept all weekend because this even happened. If I were in the field when this occurred I would have caught it, but I was not. It's still my fault for growing my business and having others do the work which I would normally would do if I was not office bound.

The foreman never looked at the nameplate......he wired up the low and high voltage sides and never thought about the XO terminal being on the wrong side. He had it inspected and they missed it......he turned on the power.....tested all wiring with a meter and moved on........there was never even a problem when they tested power at the MRI equipment.

It happened and I am dealing with it. So please do not treat me as if I am a simpleton for not completely understanding all of your explanations. I came here for info.....not to be put down and ridiculed for what happened.

My costs to fix it will never out weigh the cost for what could have happened......I just thank God nothing did happen before I discovered the issue.

$4500 TX, $20,000 x 3 days lost revenue owed to the doctor, $2000 labor and materials to replace it and $1000 per day for the cryogenic gas costs now that the chillers are down......which is a heck of lot cheaper than someone losing their life due to a mistake.

 

[GoldDigger]

Glad to hear that it is well on the way to being corrected, and with relatively small cost compared to what it might have been.

I think you should be able to complete your understanding of the problem if you analyze what I suspect, namely that the X0 terminal on the POCO side (de facto primary) is connected to ground and by virtue of that ground/EGC connection is tied back to the POCO neutral. It does not appear any different to the transformer than if the X0 had been tied directly to the POCO neutral, or tied a slight bit more indirectly through the primary EGC.
What counts is that POCO current is going into X1, X2, and X3 and is able to also complete its loop by going through X0 back to the POCO secondary(s).
If X0 had not been connected, the voltage at X0 would have floated away from ground so that the equation (X1-X0) + (X2-X0) + (X3-X0) = 0 can be satisfied.
If X0 is tied to ground/neutral, then that equation will only be satisfied if the POCO voltages are balanced.
Whether the POCO phase(s) are lost by X3 (for example) going open, or X3 going to ground, or by only one line to line phase being driven, there is no way that the transformer can see a valid input.

 

[GoldDigger]

Whatever ....

My apologies. The "you" in that post was not intended to be you.
A small confusion of posts and posters and quotes.

 

[mike_kilroy]

It happened and I am dealing with it. So please do not treat me as if I am a simpleton for not completely understanding all of your explanations. I came here for info.....not to be put down and ridiculed for what happened.

Ksmith - PLEASE DO NOT THINK ANY OF US ARE TRYING TO DEGRADE YOU! You have done nothing but been wonderful coming here for info!! MY last post is to just make if clear that the primary Xo has NOthING to do with the secondary load. Your last post made it sound like you believed it did. I just do not want YOU to misunderstand the how and why of what you have.

You have been fantastic in coming here for ideas of what is going on. You rightly realized the situation was wrong. Now, when you get into the nitty gritty of these discussions, realize us engineers and such will argue the minutiae points such as this, and it reflects not on you.

 

[ptonsparky]

Ouch. I've had to eat a screw up or two and it hurts in more ways than just cash. Prompt resolution helps. :thumbsup:

Thanks for answering all our questions.

 

[kwired]

I understand there is no current flow from the 480 winding's back to the Utility.....I stated it wrong earlier I know....I was trying to refer that wherever the imbalance was coming from that it was making it back to my EG at the MDP and failing that wire....

The phase imbalance is coming thru the XO wire we ran to the TX frame connection and back through my 208V, line side, equipment ground where it's over heating at it's termination......

Kind of skimmed through and didn't thoroughly read the last 10-15 posts at this point, but the X0 should not be connected when "backfeeding" such a transformer and if that would have been done many of the problems you are having would disappear.

Still amazed that any loads utilizing the neutral on the 480 volt side work at all. Is possible you maybe have a neutral terminal in the equipment as standard but maybe there is no 277 volt loads on this particular unit? Or if there is 277 volt equipment those particular loads must be pretty balanced, because what you have is basically no different then if you were connected to a 480/277 system but had an open circuit in the supply neutral. That point is grounded in this case, but still has no path back to the delta winding that is the source.

Add: If there would be a bond to a wye point somewhere on the system, say someone connected the center point of a wye connected motor to what they thought was a neutral for some reason, then the motor would act as an autotransformer to create a relatively reliable 277 volt neutral for anything else on the same system, as long as that motor was in operation anyway.