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Thread: Transformer Issue

  1. #1
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    Transformer Issue

    I went to a job today to check a ground current issue on a new UPS, they were measuring 15.6 amps on a ground conductor. The UPS is 480/480 3-wire with a factory supplied 480 delta to 208/120 wye 150 kva shielded isolation transformer. So I am told I could not verify the transformer type.

    There is a ground jumper from the frame of the transformer the bottom of a ground bar, from the ground bar there is a bond jumper to a Ground bar in the UPS and then from the ground bar in the UPS there is a bond jumper back to the top of the ground bar in the transformer cabinet. Current seems to be circulating on the from the transformer frame to the UPS ground bar and back to the transformer.

    No load on the secondary, Primary current 15/11/8. With 70% load on the transformer and the ground current went to 22 amps. Utilized a resistive load.

    Results were the same with the UPS on and with the UPS in bypass and the UPS off.

    Removed the top bond jumper in the transformer to the UPS ground bar and current was still on the bottom jumper in the transformer.

    The ground bars are mounted to the metallic frame of the transformer and UPS, so the effect is the same with or without the top ground jumper.

    No measurable current on the grounding electrode conductor, Neutral-Ground Bond or any EGCs
    Any thoughts?
    Brian John
    Leesburg, VA

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    Is the UPS connected to supply or checked during isolation? What are voltage readings, phase-phase and phase-ground? What currents do you get with the transformer de-energized?
    BB+/BB=?

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    I noticed you said no EGC currents so that may eliminate common mode signals.

    Check the voltages to see if you might have a partial fault in the transformer. You might meg it also.
    BB+/BB=?

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    This was brought up on another forum.

    I think the transformer may be an autotransformer


    Not sure the practical reason for an autotransformer for a voltage change of 480 to 208/120? Seems the copper and steel required would be same as a Delta Wye or close.

    Have to think about the XO would that be ground on the primary
    Brian John
    Leesburg, VA

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    Quote Originally Posted by brian john View Post
    I went to a job today to check a ground current issue on a new UPS, they were measuring 15.6 amps on a ground conductor. The UPS is 480/480 3-wire with a factory supplied 480 delta to 208/120 wye 150 kva shielded isolation transformer. So I am told I could not verify the transformer type.
    What is the 480/208 transformer for?
    Si hoc legere scis nimium eruditionis habes.

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    The UPS is 480/480 the transformer feeds the critical loads at 208/120 VAC
    Brian John
    Leesburg, VA

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    what are voltage readings. a core may be shorted to a winding

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    Primary A-B 480, B-C 480, C-A 480
    Secondary A-B 208, B-C 208, C-A 208, A, B, C to N 120
    Brian John
    Leesburg, VA

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    How is the static bypass fed? Is it 480V and fed to its SS on the primary side of the XFMR or 208V onto the secondary?

    Since you said it was a factory transformer, that why the bypass voltage is important, because as you know there is a N-G bond up at that source that is providing the needed grounding.
    Ron

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    Quote Originally Posted by ron View Post
    How is the static bypass fed? Is it 480V and fed to its SS on the primary side of the XFMR or 208V onto the secondary?

    Since you said it was a factory transformer, that why the bypass voltage is important, because as you know there is a N-G bond up at that source that is providing the needed grounding.

    The Bypass is on the primary and is 3-wire as well. Isolated the transformer from the UPS, with no connection to the UPS Bypass the issue with the transformer secondary
    still existed
    Brian John
    Leesburg, VA

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