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Two transformers 'trick' for 3 phase

Discussion in 'Electrical Engineering' started by Michael Moroney, May 25, 2004.

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  1. Question: If you need a certain amount of power (kva) 3 phase, are 2
    transformers each with half the total kVA rating cheaper than 3 each with
    1/3rd the total?

    I was thinking of the Scott-T transformer configuration, which generates
    90 degree 2 phase from 3 phase with 2 transformers. It also can produce 3
    phase from 90 degree 2 phase when used "backwards". So if you have two
    Scott-T configurations back-to-back you have 3 phase to 3 phase with 4
    transformers. Not very useful. But since one transformer simply feeds a
    second, they can be combined. So now you can have 3 phase to 3 phase with
    only 2 transformers. But they are different: Transformer 1 has primary
    and secondary A-CT-C and Transformer 2 is wired B-CT(1) (CT(1) means to
    centertap of 1) The two transformers have different voltage/current
    levels but have the same turns ratio.

    Is there any reason why this configuration isn't used for commercial power
    systems? The need for different transformers outweighs the fact you need
    only 2 instead of 3? I don't know what the minimum kVA of a pole pig
    is but I would think this combo might be useful for serving low power
    3 phase loads.

    I know about open-delta configurations that also use 2 transformers, but
    if I understand correctly they have a substantial reactive current so
    they can provide less than their rated kVA even to a 1.0 powerfactor load.
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