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Re: delta-delta, wye-wye, delta-wye, wye-delta transformer connections, which?

Discussion in 'Electrical Engineering' started by Don Kelly, Sep 9, 2004.

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  1. Don Kelly

    Don Kelly Guest

    delta delta- possible and works well- no ground can mean that the neutral on
    the secondary floats. That is not good from a safety basis. This is also
    true of a wye delta. Single phase voltages will be line to line so if you
    have a 240V line to line secondary you can handle 240V single phase loads.
    If you center tap one winding to ground you can handle 120V loads but suh
    loads are limited to one leg.

    Wye-wye is not used as there is no path for triplen harmonics in the
    magnetising current and there will be 3rd and multiples of 3rd harmonics in
    the voltage waveform. Not wanted. (where wye-wye is used, there is usually
    a tertiary winding which is in delta- this takes care of this problem)

    Delta-wye with the delta on the high side provides for a grounded neutral
    and elimination of floating voltages Single phase loads can be taken off any
    leg to neutral and can be balanced between phases- this is also an
    advantage. Thus you get 208/120V service. You can use either 3 single
    phase transformers or a single 3 phase transformer for each connection. At
    this voltage level single phase units are everywhere so that it is often
    advantageous to use 3 single phase transformers.

    Overall, except in very specific situations, a grounded secondary provides
    safety benefits.
    Unless there is a good reason to do otherwise- the delta HV/Wye LV is
    generally the best choice for this level of distribution.

    Backfeed has been mentioned (wye/delta transformer with one HV leg
    open ) -possible but probably a non -issue in general. There will be an
    associated inbalance due to impedance differences and, while there may be
    some loading on the transformers, it will not appear on the energy bill.

    For high voltage lines the HV is usually wye and the LV of the transformers
    are delta -for several advantages which result -on a delta HV, atmospheric
    charges can cause flashovers, ground faults can produce severe damage
    without detectable changes in line currents (fault fed through line
    capacitance), cost advantage to transformers as the insulation can be graded
    from hot end to ground rather than be the same throughout.

    Don Kelly

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