Xfmr Reverse Connected

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mjobee

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When a typical 150 kVA, 480V, 3PH delta to 208Y/120V, 3PH dry type xfmr is reverse connected with a 208V, 3PH, 3W primary, is there any negative impact on the xfmr because it is connected with a 3PH, 3W feed in lieu of a 3PH, 4W feed. Are there any engineering or NEC principles violated. Any thoughts.


Michael
 
This was just discussed and I don't have time to find the other post.
A step down transformer is 480 delta by 120/208 Wye.
Reversed its 208 wye by 480 delta. The secondary by 250.30 and 250.20 must have a grounded conductor. The delta would become a corner ground.
I maintain its a violation unless the transformer is listed for the reverse wiring. Sq D makes a step up transformer. Its delta by wye....
 
tom baker said:
This was just discussed and I don't have time to find the other post.
A step down transformer is 480 delta by 120/208 Wye.
Reversed its 208 wye by 480 delta. The secondary by 250.30 and 250.20 must have a grounded conductor. The delta would become a corner ground.
I maintain its a violation unless the transformer is listed for the reverse wiring. Sq D makes a step up transformer. Its delta by wye....
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I was asked this question before and quite frankly I was at a loss for words on a reply. Mr. Baker just made it very clear to me what the answer is. Thanks. I am now enlightened.
 
I agree Tom. Having established that, is a delta to delta xfmr in a typical commercial building application legal. I wouldn't think so, & IEEE standard 241, & other IEEE standards, also recommends not using the application unless special engineering considerations are used. Yet this application is frequently used for equipment hookup, ie a 480V, 3PH delta to 400V, 3PH or 380V, 3PH delta. Any Thoughts anyone.
 
Tom,
The secondary by 250.30 and 250.20 must have a grounded conductor.
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The code does not require that the secondary of this transformer be grounded. The system that supplies the transformer is required to be grounded, but there is no requirement that the grounded conductor be connected at the transformer.
Don
 
There is a problem in my opinon in having the secondary ungrounded at 480 volts. It can lead to overvoltages in a fault up to 2200 volts.
And even if grounded, its 480 to ground, not 277, leading to a higher voltage stress on conductors.
As far as the listing, it makes sense the transformer would have to shown with voltages by the mfg to be able to use it as a reverse connection. The UL white book does not mention this at all under the catagory "transformers"
 
mjobee said:
...IEEE standard 241, & other IEEE standards, also recommends not using the application unless special engineering considerations are used.
Click to expand...

The engineering considerations typically include:
Addressing the increased inrush current and probable difficulty in selecting the OCPD.
The HV will be lower than expected.
The poor performance of the taps not being on the primary side.
Dealing with all of the factors associated with grounding delta systems.
The design of the transformer windings such as T-connections and compensation windings.
Making sure the X0 terminal is not connected.

edited to add a consideration
 
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tom baker said:
I maintain its a violation unless the transformer is listed for the reverse wiring.
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Tom,
Have you ever found a written statement prohibiting the reverse connection of general purpose transformers or is it just your opinion?

I have seen manufactures saying specifically not to reverse connect some transformers (i.e. the fan cooled GE units). But every other statement has been "while not prohibited, it is not advised" primarily due to the problems with inrush currents and voltage adjustments.
 
Tom,
There is a problem in my opinon in having the secondary ungrounded at 480 volts. It can lead to overvoltages in a fault up to 2200 volts.
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Yes, there can be a problem, but that is a design issue as the code permits the 480 to be ungrounded. As far as the very high voltage it can occur under some somewhat rare conditions, and is very unlikely on a system this small.
Don
 
As far as I can tell, Jim and Don are right on target. So far I have checked
IEEE C57.12.01,
C57.105
UL1561,
5085

I even looked at NEMA TR1

There is a reference in IEEE 12.01 that if the xfmr is rated at 500kVA or more and is suitable for step-up, then the nameplate must be so marked. However, IEEE 12.01 is for HV sides 601V or greater - so it may not apply to this conversation.

I am not finding any listing issues with reverse feeding.

carl
 
tom baker said:
There is a problem in my opinon in having the secondary ungrounded at 480 volts. It can lead to overvoltages in a fault up to 2200 volts. ...
Click to expand...
The testing in UL 1561, which I think applies, lists 4000V for a 480V winding.

carl
 
tom baker said:
Jim, no.
Perhaps I'll ask Mike Holt, he may have some UL contacts
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I will be interested in the answer.

Advising against an application is not the same as prohibiting it. But allowing an application and encouraging it are not the same either.

Personally, I cannot think of very many situations where I would not encourage a 208-480Y/277 step-up transfromer instead of a reverse-connected one.
 
back feeding transformer

back feeding transformer

I recently had to backfeed a 150kva tx 208 pri. to 480 sec. After doing so and energizing the tx as suspected when checking voltage to ground there was none, on the 480 side. My concern was to clear a ground fault if it was to occur , on a phase to phase the breaker should trip.

I called the manufacture of the tx and they said to ground one of the sec delta phases, which it worked.

We could of puchased the correct tx 208 delta pri. and 480/277 sec. but the neutral wasn't needed though. The manufacture said though it was common to just ground a leg.
 
If you corner ground the secondary (phase A) of the transformer, what would happen if you had a Ph A to Gnd fault. I believe you would never open any breakers and any metal bonding would be hot. I would think you would have to install a zero sequence relay to detect an unbalance current on all three phases. Or am I missing something?
 
Bjenks said:
If you corner ground the secondary (phase A) of the transformer, what would happen if you had a Ph A to Gnd fault.
Click to expand...

That would be no different than a ground fault on the neutral of a Wye system.
 
I am not understanding your response... If you corner ground the A phase, then the A phase and ground are at the same potential. So if you have a fault on the B phase the breaker would trip, but if you have a fault on the A phase I wouldn't see how it would trip the breaker. You would have the A phase and the ground conductors now carring the current in parallel.
 
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