480V Primary 208V Secondary Transformer question

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ksmith846

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Port St Lucie, Florida
Need some help......I have a 3 phase 480V Delta Primary to 208V WYE Secondary dry transformer......however it was hooked up in reverse.....whereas the Incoming Power is 208V and the Outgoing is 480V.....It feeds a 3 phase 4 wire 480 Volt Panelboard.....they installed a neutral wire from the XO terminal, thru a lug on the frame (stripped the insulation in the lug) and terminated it in the 600V fused disconnect, which then feeds a 480V Panelboard. Al other equiment grounds are terminated in lugs on the shell interior. This cannot be a correct installation in my mind.....GE even stated that it's not correct and a neutral cannot be derived from it.

I've researched and read that it can be wired in reverse and I could derive a Neutral from the 480V side and terminate onto H2......this doesn't make sense to me and GE said it could not be done because it is now basically a Delta/Delta TX......the MRI equipment needs a Neutral and it somehow works the way it is wired right now...as long as FPL power is on. There is Generator feeding the building downstairs, but does not feed the MRI off service.....it is located in an meter room on that floor and feed from an MDP downstairs in the Main Electric Room.

This 480V Panel feeds two 3 phase rooftop Chillers and one MRI machine/equipment. The power company has lost B phase to this building's service 4 or 5 times recently.......each time this has happened the Transformer shakes so loud that no one can speak in the office. It has also caused the Equipment ground terminated in the 208V MDP to burn back the insulation. Each time this has happened the people in the office have turned off the Breaker feeding the transformer until the power company fixed it's problem. Then restarting the TX with no issues.

I am planning to install a new 208V primary to 277/480V secondary transformer to at least have the properly rated equipment.......my question is if anyone can lend me some advice as to why the transformer shakes so badly when a phase is lost and why would the equipment ground be carrying amperage and melting under this condition? Is the XO termination lost when the one phase is missing (it typically has been B phase by the way)....My guess is the Neutral feeding the 480V panel from the 208V XO has to be the root of the problem, but I do not know why.
 
ksmith846 said:
I am planning to install a new 208V primary to 277/480V secondary transformer to at least have the properly rated equipment.......my question is if anyone can lend me some advice as to why the transformer shakes so badly when a phase is lost and why would the equipment ground be carrying amperage and melting under this condition? Is the XO termination lost when the one phase is missing (it typically has been B phase by the way)....My guess is the Neutral feeding the 480V panel from the 208V XO has to be the root of the problem, but I do not know why.
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I agree with iwire, you need a properly configured transformer put in today. As to why the transformer is shaking and smoking, using the XO from the wye side of the transformer is like mixing north and south bound lanes on the interstate, they were never meant to do that and nothing but bad can result.
 
ksmith846 said:
I agree.....any other input as to what happens when the power is out?
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I am not a transformer guy, but when an electrcal device is wired horablly wrong I expect bad things to happen.

Where this is medical equipment I would fear greatly leaving it online.
 
I am most assuredly getting the correct transformer.....it arrives next Tuesday.......the MRI equipment and chillers cannot left turned off more than 2 to 3 hours....so I cannot just turn it off until then....unless I get a generator out there
 
Connecting, even indirectly through EGCs and GES, the X0 of a wye primary with a delta secondary will never work properly. The circulating currents from any supply voltage imbalance will be intolerable.

This is much worse than what would happen on phase loss alone without the transformer.
But at the same time, I would think that the cost of a phase loss detector to protect the equipment too may be justified.

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Found this on another thread.....exactly our problem....

If your transformer is, as Hurk eluded to, actually a 480 delta to 120/208 wye transformer that has been reverse fed and XO is bonded, your meltdown is definitely caused by the bond at XO. I've seen this a number of times in a string of carwashes in my area that had 480 volt equipment connected in that manner. Power company loses a phase and "poof" the smoke starts coming out of equipment. One of the building steel wires at a site was sleeved up a wall in PVC conduit and the PVC literally melted into a puddle on the floor before they called me. Square D makes a transformer and I'm sure others do as well that is designed for to be fed with the 208 on the primary and will give a 277/480 volt output on the secondary.
 
Not only must you not bond the X0 on the low voltage side, doing so does NOT make the 480V side into a Wye transformer! If it is wound as delta, no hookup method is going to change that. So if you are using anything on the 480V side at 277V (L-N), it is in mid-process of destruction right NOW. The Line to Ground (because it's not really a Neutral for that side of the transformer), might read as 277V at any given moment in time, but that is actually dependent upon the capacitance and that changes. It's a very bad situation, you may not want to wait until the new transformer arrives.

If there are no 277V loads on the 480V side it's less immediate risk, but if there is a fault it becomes one in that the OCPDs may not clear it as expected.
 
Jraef said:
Not only must you not bond the X0 on the low voltage side, doing so does NOT make the 480V side into a Wye transformer! If it is wound as delta, no hookup method is going to change that. So if you are using anything on the 480V side at 277V (L-N), it is in mid-process of destruction right NOW. The Line to Ground (because it's not really a Neutral for that side of the transformer), might read as 277V at any given moment in time, but that is actually dependent upon the capacitance and that changes. It's a very bad situation, you may not want to wait until the new transformer arrives.

If there are no 277V loads on the 480V side it's less immediate risk, but if there is a fault it becomes one in that the OCPDs may not clear it as expected.
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How much does an MRI cost? Mind boggling that someone did not catch this.
 
Sculpting63

Sculpting63

Hello. I am not an expert but I have spoken about transformers with my Electrical Engineer friend. He has told me very strongly that you never "cross a transformer with a neutral"! It sounds to me like this is what you have done.
God Bless, Keith
 
kwired said:
just many thousands more then the proper transformer to supply it costs.
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Yeah no kidding. 3 years ago I went into the ER with severe back pain and a high fever. They gave me an MRI, turned out I had a kidney stone the size of a quarter (and sepsis, which almost killed me). When all was said and done, the bill for just that MRI was $40k!
 
Interesting thread. I don't understand how the neutral is even completing a circuit back to the 480V secondary?

How would anything work on that to begin with, even with all phases present?

Any ideas what's causing the huge ground return current during phase loss?
 
sculpting63 said:
Hello. I am not an expert but I have spoken about transformers with my Electrical Engineer friend. He has told me very strongly that you never "cross a transformer with a neutral"!
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Never say never. If it's a three phase autotransformer the neutral passes through and is common to both sides.
 
Wow, Just read this thread and all I can say is WOW! I'm surprised the equipment is functioning at all and who knows what damage has already been done. The current transformer in service should be tagged for salvage as soon as its taken out of service. I'm sure the windings and laminate are close to fried.

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Without posting wiring diagrams of inside the chiller and mri, no one can say is there was ANY 480v line to XO required for balanced operation. In any case, tying the 480 XO to a completely separate pwr source like they did should not have even affected the voltages from any 480v line to the Xo. Only the 480v LOADS between 480v line and Xo would have affected the measured voltage. As jraef said, the L-Xo voltages would be random, dependent on the 480v L-Xo loads only.

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.

And the biggest issue I see is the safety issue of effectively having NO GROUND on that 480v side.

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mike_kilroy said:
Don't worry or confuse the issue saying it may be an autotransformer. An auto is Always Y:Y. Not possible to be D:Y.

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It is perfectly possible to build a D:D autotransformer. And if the input happens to be wye the output will be effectively wye grounded.
Visualize an extension (in the clockwise direction) from one end of each side of the delta triangle. The result will have the voltage you want, and will have equal phase angles, but will, I think, have a phase rotation which is related to the step up ratio that you are using. It is simple for a buck-boost but more interesting for a large step up ratio like 208 to 480.

But unfortunately the NEC does not seem to allow the use of an autotransformer unless one end of the winding is a grounded conductor.
 
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