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Thread: 240volt Delta non grounded

  1. #11
    Join Date
    Apr 2008
    Location
    Ann Arbor, Michigan
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    6,255
    171010-1416 EDT

    LarrywdH:

    Without going back thru the various posts my suggestions are:

    1. Assume that your cabinet is what you call ground (your voltage reference measuring point), and this has negligible difference in voltage relative to remote earth.

    2. Disconnect all wiring from the secondary of the problem transformer. Hopefully this can be done with a disconnect switch, breaker, or fuses.

    3. Measure your three voltages from the secondary to the cabinet. Should be moderately balanced. If not balanced, then is the unbalance about what it was with the wiring connected? This includes meaning that the same phases show about the same results.

    Report back.

    .

  2. #12
    Join Date
    Jul 2017
    Location
    Menomonee Falls, WI, US
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    gar

    If I understand you correctly, the transformer feeds a 225amp main breaker, I opened that breaker, so the transformer is energized but only feeding about 5feet into the top of an open breaker and goes no where else.
    The readings phase to phase are perfect 240v, the readings phase to ground are 131, 158, 128volts.
    I think this answers your question.
    Thanks

  3. #13
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    Apr 2008
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    171010-1457 EDT

    LarrywdH:

    Your problem is within the transformer, or the short wiing from the transformer to the breaker, or within the breaker on the input side.

    .

  4. #14
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    Apr 2008
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    171010-1526 EDT

    The next obvious step is to disconnect the wires at the transformer, and check the transformer balance to ground.

    .

  5. #15
    Join Date
    Jul 2017
    Location
    Menomonee Falls, WI, US
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    gar

    Thanks,
    I haven't seen this before, and I can't imagine what would cause what I am seeing.
    One conductor, winding, etc. has a different capacitance to ground?
    Anyone with an insight?
    I'll keep thinking/investigating.

  6. #16
    Join Date
    Apr 2008
    Location
    Ann Arbor, Michigan
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    171010-1604 EDT

    LarrywdH:

    Assuming you can now definitely say the problem is in or associated with the transformer, then it would be interesting to see if you can estimate the magnitude of the leakage impedance.

    I assumed you used a high impedance meter for your measurements, 10 megohm meter. Your readings 128, 158, 131 imply an unbalanced leakage in the transformer. Assuming an average of 139 V (calculated from 128, 158, 131) a 47,000 ohm resistor would dissipate 0.41 W. At 158 V 47k dissipates 0.53 W.

    Create a wye resistive load of three 47,000 ohm 1/2 W each resistors. Balance this load within 1%. Put this load on the transformer. This should not change any of your readings. Measure the voltage from your ground reference to the resistor midpoint.

    Next ground the resistor network midpoint to your ground reference. Measure the three leg to ground voltages. The change in the readings from 128, 158, 131 to the new readings will indicate the relative transformer leakage compared to the 47k value.

    If the readings come close to 139 V, then your leakage is much greater than 47k. If the readings don't change much 128, 158, 131, then leakage is an impedance much less than 47k.

    The capacitive reactance of 0.06 ufd at 60 Hz is around 47k.

    .

  7. #17
    Join Date
    Jul 2005
    Location
    LA basin, CA
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    1,537
    Is there a safe way to disconnect transformer power, and Megger test transformer coils to ground?
    Roger Ramjet NoFixNoPay

  8. #18
    Join Date
    Jul 2017
    Location
    Menomonee Falls, WI, US
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    22
    Gar, That with take a bit of planning, I will look into getting the materials.

    Ramsy, Yes I can megger the windings.

    Thanks

  9. #19
    Join Date
    Apr 2008
    Location
    Ann Arbor, Michigan
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    6,255
    171011-1130 EDT

    LarrywdH:

    Do the megger test first and see what that tells you.

    If you have not disconnected the wires at the transformer yet, and made the output voltage test, then do that before anything else to make sure the problem is not outside of the transformer.

    .

  10. #20
    Join Date
    Apr 2008
    Location
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    171011-1200 EDT

    LarrywdH:

    By any chance is there something like an MOV or other transient suppressor hung across the input side of the breaker, and therefore output of the transformer?

    .

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