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    Grounding Transformer

    Can someone please explain why one would want a ‘grounding transformer’? It appears it’s a Wye-Delta but nothing is hooked up on the secondary Delta side. ETA: in application it’s fed from a switchboard
    Thank you

    #2
    Originally posted by follybeacher View Post
    Can someone please explain why one would want a ‘grounding transformer’? It appears it’s a Wye-Delta but nothing is hooked up on the secondary Delta side. ETA: in application it’s fed from a switchboard
    Thank you
    Is it used for protection or ground reference? A usual fixture in a switchboard is a GPT = ground protection transformer! Its composed of three transformers connected in Wye-Delta but a relay is inserted at one corner of the delta secondary, that relay only becomes energized when one of the phase in the primary side gets grounded (broken-delta)! Most relay settings are set 10% of the possible broken-delta voltage.
    Another use of a wye-delta transformer is to have a reference ground with the delta side just left unloaded. A ground connection is derived at the primary winding neutral, while the delta acts to stabilize the voltage due to any unbalanced loading on the primary side.

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      #3
      Generally, a 'grounding transformer' is a transformer that derives a neutral which gets added to an ungrounded delta system to provide a neutral which can then be grounded.

      The requirement is that this transformer be low impedance with respect to ground faults; if you connect one of the delta phases to the neutral point, lots of current is supposed to flow, just like a fault to ground in a conventional grounded wye system.

      A set of wye coils doesn't have this required low impedance; just like an ordinary transformer you only get magnetizing current to flow in the primary if you have no current flow on the secondary. In the transformer you describe, a ground fault would cause current to flow on the wye 'primary' side and circulate on the delta side, giving the required low impedance.

      Another option for a grounding transformer is a 'zig-zag' connection, where your 'wye' is broken up into two coils per leg, and half of each phase sits on a transformer core with half of another phase. There is no need to go into the details of how this works, the key point is that it permits lots of current to flow from a faulted phase through the grounded neutral point.

      -Jon

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        #4
        Originally posted by topgone View Post

        Another use of a wye-delta transformer is to have a reference ground with the delta side just left unloaded. A ground connection is derived at the primary winding neutral, while the delta acts to stabilize the voltage due to any unbalanced loading on the primary side.
        One way to think how this wye-delta works is the following: The voltage across each winding of the delta secondary is proportional to the voltage across the corresponding winding on the wye primary. The delta has all of its windings in series to make a closed circuit loop, and so all of the winding voltages sum to zero via Kirchoff's voltage law. Now as long as the voltages on the wye are balanced then so are the delta voltages. With balanced 3-phase voltages the sum of all three voltages is zero, and so in this case the voltages across the delta windings would sum to zero even if the loop was broken. Therefore if the voltages are balanced then no current will flow even when the delta loop is closed. However, if there are loads that tend to unbalance the voltages on the wye, then as Jon mentioned this will induce a circulating current in the delta loop. This delta current will be reflected back as a common current that flows through all three of the windings of the wye and which compensates for the unbalanced load current, improving the balance of the primary voltages relative to the neutral. As a result this also creates a lower impedance for "zero sequence" load currents and ground faults than a wye without a delta would, as Jon mentioned above.

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