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Thread: Voltage Unbalance Reason

  1. #1
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    Voltage Unbalance Reason

    dear All,

    Hi, I am having a problem of unbalanced strange values of system voltages in of our power generation plant,
    the disturbance in voltage is being reflected on Low voltage side only,

    system diagram is attached.

    Attachment 20244

    what the system must show at low voltage side is;
    1. P-P 480V
    2.P-N 277V

    but the current scenario on delta side of transformer is;

    L1-L2= 316 V
    L2-L3= 147 V
    L3-L1= 221 V

    L1-N= 594 V
    L2-N= 599 V
    L3-N= 530 V

    neutral is kept floating on both sides, i.e. both LV & MV neutral is floating.
    is there any fault in on of the system phases?

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    Your attachment didn't work.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

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    please see attachment for the said problem
    Attached Images Attached Images  

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    I would guess the Neutral and ground are bonded at the gens or at the parallel bus, right. I would check there first.
    Ron

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    neutral is kept floating, not connected.

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    There seem to be two separate problems, unless you can somehow identify a coupling between the two.

    1. The relationship between the P-N and P-P voltages is not possible for a three phase system with equal 120 degree rotation from phase to phase. If you draw the phasors out on paper with scaled lengths I think that you will find they match if you have roughly 550V for each P-N but one of the three windings of the star is reversed in polarity. That will leave the P-P voltages smaller than the P-N voltages and will have one P-P voltage approximately twice the other two, since the end points of the three phasors are closer to a center tapped single phase system than to a proper star.
    Possibly the leads were incorrectly identified when the generator was manufactured or rewound or else somebody simply miswired them.
    The problem would be obvious if you were able to look at the voltages on an oscilloscope (always triggered off one phase) instead of just measuring the amplitude with a meter.

    2. The P-N voltages are far too high. This might be explained by the error in 1 wreaking havoc on the operation of the voltage regulation system.
    Last edited by GoldDigger; 05-01-18 at 09:09 AM.

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    Confirm that all generators have the same pitch; i.e. all 2/3 or all 5/6 pitch machines; but not mixed.
    "Just because you're paranoid, doesn't mean they're not out to get you"

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    Ungrounded Neutral Symptions

    Is this a new installation? If not has there been any electrical work (PM/ routine NETA testing, equipment replacement/ rewiring, etc) been done in the problem area lately? Has it ever operated properly?
    I agree that the P-P voltages on the Delta winding are a problem. What is confusing is you state neutral is kept floating on both sides, i.e. both LV & MV neutral is floating. But since the gen winding is Star connected then by code the Xo must be grounded at the gen for proper voltage output. If this connection is loose or non-existent then the P-P and P-N voltages would be unstable and all over the place just as you show.
    As Ron stated earlier I would check this connection on both gens.
    Were these measurements taken with the gens in parallel or separately? I would find it hard to believe they would even parallel with these voltage readings.
    The only other area I would suspect are the gens voltage regulators. Load bank testing would, for sure, weed these out.
    Am I in shape?? I get plenty of exercise pushing my luck!!

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    was it running normally before and this problem just occured?
    or was this identified during commissioning?

    L-N ~ 550-600
    Ph-Ph 150-300
    ???
    not sure how that can be

    has each gen been isolated/measured?
    have the gen winding connections been verified?

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    Quote Originally Posted by Ingenieur View Post
    was it running normally before and this problem just occured?
    or was this identified during commissioning?

    L-N ~ 550-600
    Ph-Ph 150-300
    ???
    not sure how that can be

    has each gen been isolated/measured?
    have the gen winding connections been verified?
    See post #6 for how it can be.

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