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Thread: Delta-Delta Tranformer vs. Delta-Wye

  1. #1
    Join Date
    Jul 2008
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    11

    Question Delta-Delta Tranformer vs. Delta-Wye

    A little while back, I was putting in a delta-wye transformer (480V in, 480/277V out) as an isolation transformer to lower the available fault current to an air compressor with a VFD. Someone suggested that it really should be a delta-delta transformer in that application, but since we had a delta-wye on hand, it would be fine.

    Anyone know what the advantages would be for a delta-delta? I always thought that delta-wye was preferred for industrial applications because you would be able to ground the wye and limit the voltage to ground to 277V in this case. Would it deal with harmonics better since it is powering a VFD?

    Thanks

  2. #2
    Join Date
    Feb 2003
    Location
    Wisconsin
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    Do you require no phase shift for the line to load of the transformer?
    If no, then you can use a wye. A delta provides a zero phase shift where a wye is 30deg (Dyn1).

    Are you grounded it?
    If yes, the a delta would be corner grounded meanind the line to ground voltage would be the same as the line to line voltage.
    Where with a wye, you ground the X0 and the phase to ground voltage would be the line voltage devided by the square root of 3 which is much lower.
    Also, you have to be carefull of the ratings of the down stream devices.
    I prefer a wye because of grounding. The transformers should be identical in price so cost should ne be an issue.

  3. #3
    Join Date
    Feb 2003
    Location
    Wisconsin
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    I see no advantage to a delta-delta transformer for your application.

    While you do not need all of the features of a true "drive isolation" transformer, delta-wye is definitely easier to find as off-the-shelf units.
    Just because you can, doesn't mean you should.

  4. #4
    Join Date
    Apr 2006
    Location
    Portland, OR
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    2,338
    IMHO a Delta-Wye transformer is to be preferred for this application. With a delta secondary you don't have a good point to ground.

    -Jon

  5. #5
    Join Date
    Jul 2008
    Posts
    11
    "Do you require no phase shift for the line to load of the transformer?"
    I am fine with a phase shift, so a delta-wye should be fine.

    "Are you grounded it?"
    Yes, it needs to be grounded. And as of yet, my location does not have any corner grounded delta systems, so I would hesitate to start using them without a really good reason.

    So, I agree with you all that Delta-Wye would be preferred.

    Thanks for the responses.

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