Wye/Delta Connection

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inspector141

inspector141

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Location
Westminster, MD
A contractor phoned me and asked if this is acceptable:
Building service is 120/208 , 3 phase. He wants to step up to 480 volt to feed some compressors via a 150 kva x-former. The electrician wants to reverse feed a Delta/Wye xformer(120/208 primary/ 480 volt secondary).

Question 1: Can it be wired exactly the opposite of a typical 480 Delta primary to 120/208 Wye secondary?

Question 2: If yes, how will the secondary open if a phase to ground short occurs on the secondary?

I would appreciate some help on this matter.

Marty
 
1. Not exactly. Do not land the neutral-point of the 208-Y now-primary; connect it as if it had a Delta primary.

I learned that one here. :smile:

2. If you want the new Delta secondary grounded, you'll either have to ground one phase or use the zig-zag method.

Otherwise, you could leave it floating and use ground detection.
 
Larry,

So he could ground B phase on 480 volt sec to XO and identify with white tape, then from XO, ground to building electrodes as required?
Connect the 208 volt primary as a Delta Connection(no nuetral to XO)

He does not want to use ground detectors.
 
I'd suggest searching this forum for past discussions of reverse connected transformers.

My understanding is that you _can_ do this, but that it introduces significant problems that you need to deal with, and that using the proper transformer in the first place probably makes more sense.

1) Secondary grounding. You can 'corner ground' a delta secondary, but now your secondary OCPD must be rated for the full 480V, not for 480/277V.

2) Transformer taps. You want your taps on the _primary_ side, so that you can adjust the magnetization of the core. If the incoming voltage is higher than 208V then the transformer core may saturate, causing losses and harmonics and heat. With taps on the primary side you could compensate for this by adding primary turns. But with a reverse connected transformer, you don't have the needed taps.

3) Inrush current. When the transformer is first connected to its supply, there can be a tremendous current spike as the core builds up flux. Transformers are optimized to reduce this inrush when current is supplied to the _primary_, and will have higher inrush current when reverse connected. This may or may not be a significant problem in any given installation.

Have you considered using 'buck boost' transformers to get to 240V, followed by reconnecting the motors for 240V? Lots of reasons for and against this approach, but one to consider.

-Jon
 
inspector141 said:
... ground B phase on 480 volt sec to XO ... then from XO, ground to building electrodes as required? ...
Click to expand...
Wouldn't this make a solid connection from the transformer 208/120 neutral point to the service 208/120 neutral?

inspector141 said:
...Connect the 208 volt primary as a Delta Connection(no nuetral to XO) ...
Click to expand...
As noted, the solid bond from XO to building steel is a solid connection to the service neutral

inspector141 said:
... He does not want to use ground detectors.
Click to expand...
Why not?

carl
 
coulter said:
Wouldn't this make a solid connection from the transformer 208/120 neutral point to the service 208/120 neutral?


As noted, the solid bond from X0 to building steel is a solid connection to the service neutral
Click to expand...

You are correct the X0 bushing on this transformer needs to be left floating.

Connecting a corner of the secondary (usually B phase) to the service neutral is a different issue (it should be to the service ground).
 
I would not rely solely on the info gleaned from this site. The manufacturers have a ton of info on their sites in relation to back feeding the transformers.
I would email the manufacturer of your transformer with the model number and the specific info of what you are trying to accomplish. The tech departments respond in a timely fashion with this type of info.
 
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