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Thread: Importance of floating neutral on the primary of a wye - delta transformer

  1. #1
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    Importance of floating neutral on the primary of a wye - delta transformer

    Can someone please explain to me why it is important to leave the primary wye neutral connection of a wye-delta transformer floating rather than grounding it. I believe this has something to do with letting the primary voltage float or shift for unbalanced loads on the delta secondary? Can someone please explain how this works or give an example?

    Can someone also please explain how this connection helps voltages shift for a L-G fault on the the delta secondary?

    Any good references for this?

  2. #2
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    I tricked Google into giving me the answer. I started out with Google, Image, I used your own statement but added one item (well two terms independently).
    Frankly I was happy to see both as useable inputs! but between the two different results...

    Define; define neutral connection of a wye-delta, here are the results.

    &

    Explain, explain neutral connection of a wye-delta, (while still in Image) the fourth one from the left has a good fresher of 3 phase circuits and talks of your peril to understanding what is happing based on your OP question. It is very simple explaination, but hopefully else from the links the light we be sought else where.

    The Same link just as described is Here and a little over half way down the web page it explains what is happening!
    Last edited by cadpoint; 02-21-11 at 06:12 AM.
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    Quote Originally Posted by mull982 View Post
    Can someone please explain to me why it is important to leave the primary wye neutral connection of a wye-delta transformer floating rather than grounding it. I believe this has something to do with letting the primary voltage float or shift for unbalanced loads on the delta secondary? Can someone please explain how this works or give an example?

    Can someone also please explain how this connection helps voltages shift for a L-G fault on the the delta secondary?

    Any good references for this?
    ...one day a few years ago I wired a 480 to 208/120 V, Delta primary
    Wye secondary transformer backwards for a temp 208V in and get 480V out...
    (it was just for a few hours so the wires were on the floor, a wooden floor)

    When we turned this on the ground wire (connected to X0 of the 208V side)
    literally disappeared in a puff of smoke...nothing left but a black line on the floor where it had been...no fuses blew just no more ground wire. Recall it was a 75 kVA XFMR and the grnd wire was a #6 copper.

    explanation: in the first 60th of a second the frequency is essentially "1 Hz", an inductor or winding reacts like a short circuit...in this case, three windings - three short circuits. If you feel like doing the math it will surprise you.

    Sparky, EC,PE,CO

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    Whatever works on an instantaneous basis has to also work on a steady state basis. Therefore, this method should work as an easy way to analyze three phase Y connected sources using resistive loads.

    For phase voltages of 120 volts, the reference phase instantaneous peak voltage is 120x1.414xsin(0) = 0 volts, the next phase voltage at that same instant is 120x1.414xsin(120) = 147 volts and the last phase voltage is 120x1.414xsin(240) = -147 volts.
    Now the network can be analyzed using two batteries: the first battery [reference phase] can be replaced by a short circuit, the second has the negative terminal at the center of the Y and last battery has the positive terminal at the center of the Y.

    If this method is correct it should confirm the results of the link posted above and the real world results. Otherwise, analyzing these multiple phase circuits is a PITA.

  5. #5
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    Quote Originally Posted by mull982 View Post
    Can someone please explain to me why it is important to leave the primary wye neutral connection of a wye-delta transformer floating rather than grounding it. I believe this has something to do with letting the primary voltage float or shift for unbalanced loads on the delta secondary? Can someone please explain how this works or give an example?

    Can someone also please explain how this connection helps voltages shift for a L-G fault on the the delta secondary?

    Any good references for this?
    Actually, I was of the impression it should be grounded... but it is only an impression. I am interested to know myself, either way, and/or the specifics if it should be one way and not the other dependent on conditions.

    Ran across the reference: The Whys of the Wyes, a long-standing GE publication. Don't have time to read comprehensively at present. In glancing through it, I believe in the last section it recommends groundiong the neutral if at all possible...

    PS: Does anyone know why links here by default are nearly indistiguishable from regular text???
    Last edited by Smart $; 02-21-11 at 11:59 PM.
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  6. #6
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    I also have never been told a scientific reason why not to bond the X0 when a Y is used as the primary side of a transformer, the closest I have been told was that when using a common 3 legged core transformer eddy currents will build up and over load the X0 and the grounding conductor and cause the transformer to over heat if the X0 is bonded back to a Y supply, this is if the secondary is unbalanced.

    I have seen it happen many times just never told the real reason why? at least scientifically
    Wayne A. From: N.W.Indiana
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    Just don't be fairly safe

  7. #7
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    [QUOTE=Smart $;1286956]Ran across the reference: The Whys of the Wyes[QUOTE]

    Thank's read a little of it, need to hit the bed so I read it another time.
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

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    ...do the math...

    I wanted to amend my previous comment...

    As I recall back in college modeling the response of inductors and capacitors from
    time "zero" to steady state the impedance (Z) of a pure inductor is modeled by
    Z = 2(pi)f L where "f" is the frequency and L is the inductance in Henry's...

    At t = 0, f=0, so Z also equals 0...if you apply DC to an inductor is looks very much like a short circuit (ignoring the internal resistance that the wires and so forth have) so a transformer connected in a Wye is essentially three large parallel loads phase to ground. At t = 0 the current through each winding looks like a short circuit and is limited only by the internal winding resistance...

    The amount of instantaneous current is huge...

    So, never ground X0 when connecting a Delta Wye backwards...
    (and I apologize for all that I have forgotten since College!)

    Sparky

  9. #9
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    IEEE C57.105 talks a little about this:

    Certain connections or winding configurations offer low impedance to the flow of zero-sequence currents. If the
    connection is such as to permit this condition on the primary side, the transformer will supply part of the unbalanced
    phase-to-neutral loads and line-to-ground fault currents. The transformer bank is said to be a ground source. Such
    connections on Y-grounded distribution primaries are a hindrance to providing reliable and sensitive ground-fault
    protection at the source substation. Furthermore, such transformer banks are subject to serious overloads under certain
    open-conductor fault conditions (see Section 5.). The YG-D connection is a ground source on the primary side. The
    grounded T winding is similar to the grounded zigzag connection in that the ground source capability is inherent in the
    grounded winding alone.
    This has been happening every so often for the utility I work for. We used to buy two primary bushing single phase transformers (one bushing for the hot, one for the neutral). The primary neutral bushing is not connected to the primary neutral conductor (left open through an open cutout) when 3 single phase transformers are banked together in a wye-delta arrangement and not grounded.

    We are now buying single primary bushing single phase transformers and the neutral winding is internall bonded to the can. If one of these new single bushing transformers replaces a two bushing transformer in an existing bank, the primary neutral is now grounded.

    Most often you just get a blown fuse on the transformer primary when there is a fault on the utility system, but sometimes service wires get cooked as well.

  10. #10
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    Quote Originally Posted by SparkyHC View Post
    I wanted to amend my previous comment...

    As I recall back in college modeling the response of inductors and capacitors from
    time "zero" to steady state the impedance (Z) of a pure inductor is modeled by
    Z = 2(pi)f L where "f" is the frequency and L is the inductance in Henry's...

    At t = 0, f=0, so Z also equals 0...if you apply DC to an inductor is looks very much like a short circuit (ignoring the internal resistance that the wires and so forth have) so a transformer connected in a Wye is essentially three large parallel loads phase to ground. At t = 0 the current through each winding looks like a short circuit and is limited only by the internal winding resistance...

    The amount of instantaneous current is huge...

    So, never ground X0 when connecting a Delta Wye backwards...
    (and I apologize for all that I have forgotten since College!)

    Sparky
    ...but what you are saying about Z=0 and f=0 at t=0 when modeled by
    Z = 2(pi)f L is also true for a delta-connected primary...
    I'll never get there. No matter where I go, I'm always here.

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