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Applying Same phases to a 3 phase transformer

  1. Aug 9, 2012 #1
    while reading about sequence networks, I realized this. Suppose I apply same phase to the primary of Star-Delta transformer, say instead of applying A,B, and C phase to the three terminals, I mistakenly apply A,A and A phase to all the terminals, <Neutral is applied appropriately>.
    Then since the voltages applied comprises zero sequence voltage, it will also induce zero sequence voltage in the secondary, and since the delta loop provides path for zero sequence current, circulating currents should flow.
    My guess is, it should be devastating to the transformer.
    Am I right?
    Has anybody done this mistakenly?
  2. jcsd
  3. Aug 9, 2012 #2
    A transformer with a shell type core will operate at 1/3 rated voltage OK. Above 1/3 rated voltage the core will saturate and large currents will flow and there will be high flux leakage. At rated voltage it will be a race between the transformer tank/or case melting and the winding melting.

    In transformer with a core type core (No external legs), it will be a race between the transformer tank/or case melting and the winding melting.

    Above assuming that the circuit breaker or fuse doesn't open.

    Currents will flow in transformer primary.
    Current in transformer secondary will be low unless secondary is shorted.
  4. Aug 9, 2012 #3
    Thanks Carl Pugh,
    I think,the issue you are referring arises in 3-limbed transformer which has no return path for magnetic flux. In normal operation, the three fluxes in 3 limbs cancels each other so no return is required. But when same phase is applied to all, they don't cancel but instead add up, and the total flux needs to return through the air medium, there by greatly increasing the reluctance.
    This results in huge magnetizing currents being consumed.
    But with transformer with 4 legs (is this what you call 'shell type'), there will be a return limb for the flux, but since its not adequately sized, if we apply full voltage, then due to saturation, both huge magnetizing current will be consumed (although less than previous case) and huge core loss in the limb will occur as well.

    Really thanks for bringing those issue to light.
    But my original curiosity was referring to this
    As opposed to what you said, Currents should flow in the secondary as well if the secondary is delta connected.
  5. Aug 10, 2012 #4
    You are correct.
    The transformer secondaries are short circuited.
    There should be a file attached, that shows how I came to this conclusion.

    Attached Files:

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