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Thread: HELP NEEDED FOR XFMR PROBLEM (Inrush?)

  1. #11
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    I'm a slow poster - could be all this has been covered
    Here is what I translated:

    • System is a 225kva, 208/480 fed from the 208 side, powering a 480V compressor. Feeder is 400A CB. Secondary disconnect (480V) is 200A fused disconnect.
    • A/C faults on "Over Amperage". No tripped CB, no blown fuses.
    • Transformer is changed. 208 feeder CB is changed
    • I not sure exactly what was megged.

    Transformer primary to ground (208V side) ?
    Transformer secondary to ground (480V side) ?
    Transformer primary conductors ?
    Transformer secondary conductors (transformer to 200A disconnect) ?

    I tried to reengage breaker with no load on transformer and it tripped instantly twice.
    So secondary disconnect (200A fused disconnect) was open?

    so I cycled the breaker a 3rd time and I had an amprobe on 1 leg and the amp meter reading a different phase on switchgear (rest of plant is dead, so it's only reading our transformer setup) the breaker did not trip instantly, and instead for ~10 seconds there was transformer noise and both my amprobe (1,000 amp limit) and the amp meter (2,000amp limit) were maxed, I waited for Inrush to settle and feel as though I waited ample time, after the 10seconds I tripped the breaker manually, for fear it wasn't going to trip, and amp meters we're still maxxed.
    I'd say there are two problems:
    The first one is with the compressor/drive, down stream of the 200A fused disconnect. Changing the transformer/CB didn't fix that.

    Second issue is with the new transformer.

    1. Different brand/type that has different inrush characteristics,
    2. wired wrong,
    3. Not the same transformer.


    Some notes on transformer inrush, xfm feeder design:
    According to a GE paper, the inrush current for a back fed 208Y/480D transformer can be 37X the FLA. For a 225kva xfm, the 208V side FLA is 625A. Inrush current as much as 23KA. Yep, 23,000A. Expected duration is 100 milliseconds.

    The transformer that should have been speced is 208D/480Y. The 208V side FLA is still 625A, however, the inrush is 11X FLA = 6700A.

    Normal xfm feeder CB for a 225KVA, 208V primary is FLA x 1.25 = 625 x 1.25 = 781A, next size up, 800A.

    I'm a bit surprised it ever worked with a 400A feeder CB.

    Now for a bunch of wild guesses:
    The old xfm was a 208D/480Y.
    The new transformer is 208Y/480Y

    The new transformer is wired wrong. Two instantaneous trips of the 400A CB is normal (what I would have expected) The CB holding to ~10 seconds, with the measured currents above 2000A, before being manually opened, is bad news (X 2). The inrush current won't last longer than 100 ms. And, the CB should have tripped as it did the first two times.

    So, what to do?

    1. Check the old transformer nameplate against the new transformer nameplate. If the old one s a 208D/480Y and the new one is 208Y/480D. That would explain the high inrush, but not the sustained high current. If so, then .....
    2. There might not be anything wrong with the old CB or the old xfm.
    3. Check the transformer internal wiring and external wiring. As Winnie said look for a 208V WYE neutral being grounded. It could be even internal to the xfm.
    4. Borrow/ rent some additional test gear - might even have to rent a tech, in fact you may want to rent outside expertise. Check transformer turns ratio (both transformers). Check the CB contact resistance. Check the CB trip unit. (Both CBs) The new CB could be bad since it could have opened on instantaneous at 20KA+ twice.
    5. After you determine the cb is good, the transformer is the right one (hopefully a 208D/480Y, the xfm is wired correctly, then ...
    6. Get a Big CT, say something in the 5000:5 range, one CT on one 208V phase is sufficient. Just put a loop on the CT secondary. Put your amprobe on the loop, and set for peak hold.Open the 480V, 200A fused disconnect. Energize.


    Check the xfm nameplates and post back - we can go on from there.

    the worm
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  2. #12
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    From past experience I'd put my money on:
    Check the transformer internal wiring and external wiring. As Winnie said look for a 208V WYE neutral being grounded. It could be even internal to the xfm.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  3. #13
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    Quote Originally Posted by augie47 View Post
    From past experience I'd put my money on:
    Check the transformer internal wiring and external wiring. As Winnie said look for a 208V WYE neutral being grounded. It could be even internal to the xfm.
    Me too.

  4. #14
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    OP states no external connection to X0, but it is certainly possible that it is bonded to case/EGC internally.

    Sent from my XT1585 using Tapatalk

  5. #15
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    Quote Originally Posted by iceworm View Post
    I'm a slow poster - could be all this has been covered
    Here is what I translated:

    • System is a 225kva, 208/480 fed from the 208 side, powering a 480V compressor. Feeder is 400A CB. Secondary disconnect (480V) is 200A fused disconnect.
    • A/C faults on "Over Amperage". No tripped CB, no blown fuses.
    • Transformer is changed. 208 feeder CB is changed
    • I not sure exactly what was megged.

    Transformer primary to ground (208V side) ?
    Transformer secondary to ground (480V side) ?
    Transformer primary conductors ?
    Transformer secondary conductors (transformer to 200A disconnect) ?


    So secondary disconnect (200A fused disconnect) was open?



    I'd say there are two problems:
    The first one is with the compressor/drive, down stream of the 200A fused disconnect. Changing the transformer/CB didn't fix that.

    Second issue is with the new transformer.

    1. Different brand/type that has different inrush characteristics,
    2. wired wrong,
    3. Not the same transformer.


    Some notes on transformer inrush, xfm feeder design:
    According to a GE paper, the inrush current for a back fed 208Y/480D transformer can be 37X the FLA. For a 225kva xfm, the 208V side FLA is 625A. Inrush current as much as 23KA. Yep, 23,000A. Expected duration is 100 milliseconds.

    The transformer that should have been speced is 208D/480Y. The 208V side FLA is still 625A, however, the inrush is 11X FLA = 6700A.

    Normal xfm feeder CB for a 225KVA, 208V primary is FLA x 1.25 = 625 x 1.25 = 781A, next size up, 800A.

    I'm a bit surprised it ever worked with a 400A feeder CB.

    Now for a bunch of wild guesses:
    The old xfm was a 208D/480Y.
    The new transformer is 208Y/480Y

    The new transformer is wired wrong. Two instantaneous trips of the 400A CB is normal (what I would have expected) The CB holding to ~10 seconds, with the measured currents above 2000A, before being manually opened, is bad news (X 2). The inrush current won't last longer than 100 ms. And, the CB should have tripped as it did the first two times.

    So, what to do?

    1. Check the old transformer nameplate against the new transformer nameplate. If the old one s a 208D/480Y and the new one is 208Y/480D. That would explain the high inrush, but not the sustained high current. If so, then .....
    2. There might not be anything wrong with the old CB or the old xfm.
    3. Check the transformer internal wiring and external wiring. As Winnie said look for a 208V WYE neutral being grounded. It could be even internal to the xfm.
    4. Borrow/ rent some additional test gear - might even have to rent a tech, in fact you may want to rent outside expertise. Check transformer turns ratio (both transformers). Check the CB contact resistance. Check the CB trip unit. (Both CBs) The new CB could be bad since it could have opened on instantaneous at 20KA+ twice.
    5. After you determine the cb is good, the transformer is the right one (hopefully a 208D/480Y, the xfm is wired correctly, then ...
    6. Get a Big CT, say something in the 5000:5 range, one CT on one 208V phase is sufficient. Just put a loop on the CT secondary. Put your amprobe on the loop, and set for peak hold.Open the 480V, 200A fused disconnect. Energize.


    Check the xfm nameplates and post back - we can go on from there.

    the worm
    Will do. I will post back in morning. Thanks so much everyone!!!

  6. #16
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    Quote Originally Posted by Captain Planet View Post
    Will do. I will post back in morning. Thanks so much everyone!!!
    You're welcome. But - free internet advise is likely worth what you paid for it.
    And nothing says I'm not complete all wet.
    Without data you’re just another person with an opinion – Edwards Deming

  7. #17
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    Quote Originally Posted by GoldDigger View Post
    OP states no external connection to X0, but it is certainly possible that it is bonded to case/EGC internally.

    Sent from my XT1585 using Tapatalk
    Have you even seen an internally bonded X0 on a standard 225kva transformer?
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

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