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Figuring Utility Contribution

Discussion in 'General Discussion' started by bruinfan, Nov 21, 2009.

  1. bruinfan New Member

    I am new to this forum and have a question regarding utility contribution. I am performing an arc flash study and I have one utility feed that supplies 4 transformers (one is 500kVA and three are 1500kVA at 12,500:480/277V).

    I am using SKM as my software and it requires to fill in fields for the utility contribution. I have been in contact with the utility company and they will not give me any information on this. They did give me the fault currents for each transformer...
    500kVA = 32,913A
    1500kVA = 30,219A (per xfmr)

    In order to run the short-circuit analysis I made a few assumptions for the utility contribution.

    1) I assumped a 100 MVA bus
    2) I assumped a x/r ratio of 15

    This leads me to my question. Can I do this? Can I make this assumptions and get good results? Is there a better way to do this?

    Thanks in advance for anyones help on this.
    bruinfan, Nov 21, 2009
    #1

  2. acobb Well-Known Member

    Can you get to the transformers to see the impedance? Some have external nameplates if they are padmounts. That would tell you if you have been given infinite source values and you can adjust.
    acobb, Nov 21, 2009
    #2

  3. bruinfan New Member

    I can not get to that information. I should be able to figure out the impedance if I know the fault current, right?
    bruinfan, Nov 21, 2009
    #3

  4. acobb Well-Known Member

    Yes, if they gave you infinite source looks like about 1.8+% on the 500 (seems low) and about 6% on the 1500's. You could take that but I would also look at the system with about 20 kA on each unit to be safe. X/R of 15 seems a bit high. I would use 10 or less.

    Seems odd that the fault duty on a 500 would be more than the 1500 from the same source.
    acobb, Nov 21, 2009
    #4

  5. John Perrotti Member

    Utility Information

    I would be a little more insistent with the utility company. Explain to them the reasons for the information, and they should give it to you. Keep going up the ladder until someone listens.
    John Perrotti, Nov 22, 2009
    #5

  6. brainfiller Administrator

    Source Data - Last Resort

    If all else fails, here is the last resort method.

    Assuming there is a source transformer, begin with the infinte bus short circuit current. This may or may not be the worst case for arc flash so more analysis is needed.

    Next, continue the analysis and reduce the short circuit current from the maximum infinite bus value by increments of 5 to 10% and re run the study each time. If the study is conducted with one of the commercially available programs, this should be pretty easy.

    At each incremental drop, check the HRCs to see if any location jumps from category 0, 1 and 2 to 3, 4 or more.

    Keep lowering the source until HRCs begin to change and become too big. If a jump in HRC occurs by using this method, it is almost always going to be due to the fault current falling below the instantaneous trip. See if lowering the trip setting will bring the HRC back down. Perhaps offer it as a maintenance (temporary) setting.

    Finally, document how low the fault current can be before trouble develops.

    This method does not give you an answer for your specific utility condition since the utility data is unknown but it does give you a range of fault currents where you have certain amount of confidence in your HRC levels.

    The lower you can go with the fault current (greater the range of fault currents) the more confidence you can have in the results.
    brainfiller, Nov 22, 2009
    #6

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