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Thread: Primary/secondary current in a delta/wye transformer

  1. #1
    Join Date
    Feb 2016
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    Louisville KY
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    Primary/secondary current in a delta/wye transformer

    Sorry if this is the wrong forum to post this in but I figured it's a calculation question. I'm studying for the PE and going through the NCEES prep exam. Here's a question for which I'm struggling to understand their answer: https://i.imgur.com/gIJfp3O.jpg
    The solution indicates that merely multiplying the transmission line current by the transformer turn ratio (132/13.2) gives you the generator load current (760A). I don't think this is right- wouldn't it be the transmission line current multiplied by SQRT(3) AND the turns ratio because it would be converting from wye to delta? Otherwise, simply multiplying by the turns ratio would only give you the phase current on the delta side, not the line current? My research and calculations all come out to answer B.

    Here's some sources on Delta/Wye conversions as well that I'm using to support my argument:

    https://www.myodesie.com/wiki/index/returnEntry/id/3035
    https://electrical-engineering-porta...er-connections

    I'd really love if someone proved me wrong because there's nothing worse than being misled by an incorrect answer

  2. #2
    Join Date
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    Springfield, MA, USA
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    The 13.2:132 is the transformer line-line voltage ratio, not the coil turns ratio.

    As you note, with a delta:wye transformer you have a sqrt(3) term to deal with, converting the line-neutral voltage of the wye secondary to a line-line voltage. But in the case of the single line you show, that calculation is already folded into the voltage ratio shown. (For example, a 480V delta to 208/120V wye transformer as a coil turns ratio of 4:1, but in a single line it is shown as 480:208.)

    -Jon

  3. #3
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    Quote Originally Posted by winnie View Post
    But in the case of the single line you show, that calculation is already folded into the voltage ratio shown.

    -Jon
    This is what I was getting hung up on, thank you! I was right but also wrong. In your experience do single lines always incorporate that calculation?

  4. #4
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    I don't have enough experience with one-lines to give a good answer,

    However it is nearly universal to report 3 phase line-line voltages when describing 3 phase systems.

    -Jon

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