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Utility Data

Discussion in 'System Modeling and Calculations' started by mountaineer, May 18, 2010.

  1. mountaineer Member

    New to Etap and ArcFlash forum. I requested 3phase rated KV, MVA short circuit and X/R from the utility for input to Etap. The info I received is 3PhLLL - 3470A, 3PhGnd - 1989A, X/R - 2.87. This is the standard info they supply for arc flash requests.

    Not sure where to go from here. From what I've read here, a common assumption for X/R is between 12 and 15 so the 2.87 seems low.

    I was also told the above data is to the 12kv disconnect on the utility pole before the utiltiy owned pole mounted transformer (12kv to 480v). Do I now add the utility transformer and aerial line to the meter to my model?

    Thanks for any help.
    mountaineer, May 18, 2010
    #1

  2. brainfiller Administrator

    The short answer is Yes - these two articles might help:

    Transformer and Source Impedance

    X/R Ratio

    You also might want to experiment with alternate cases of lower short circuit current, say 90% and 80% of the utility short circuit current "just for kicks". This is in case the utility does any switching that can reduce the current. This will help to see if the lower current causes any excessively long clearing times that can increase the incident energy.

    Good Luck - stop back if you have any more questions.
    brainfiller, May 18, 2010
    #2

  3. mountaineer Member

    Thank you for the reply. I have those articles in my hands:). Is the utility likely to give me more info on their transformers or am I save using typical data? I have over 160 stations to do and am concerned about pushing my luck in pestering for detailed info. What would you suggest here?
    mountaineer, May 18, 2010
    #3

  4. brainfiller Administrator

    That is going to be a difficult one. You can not really assume the %Z since it is the most significant piece of data in the calculation. If the utility can not provide this, see if they can give you a range of what they use and try the opper and lower end to see what happens.

    Also... an old trick... use a pair of binoculars or camera with a zoom to see if you can capture the nameplate info off of the tranformer on the pole.
    brainfiller, May 18, 2010
    #4

  5. jghrist Well-Known Member

    You will also need the X/R for the transformer. X/R of 2.87 is reasonable for the 12 kV primary, but the X/R for the transformer will probably be higher.

    The tricky part may be getting primay protective device data from the utility. There probably is no secondary protection, so you need the TCC for the primary device to determine fault clearing time.
    jghrist, May 19, 2010
    #5

  6. Wilson99 New Member

    I requested availabe fault current from a utility for an arc flash study. Their response was for liability reasons they only give out maximum available fault current. Furthermore, they base this number on infinite on the primary (typical for a utility) but also an impedance of only 1.1%. (Surprisingly he did not base it on a transformer larger that what they currently have installed.) So for a 300kVA 208V transformer, 75kA! More typical impedances (5%-2%) result in 16kA - 42kA. Big differnce. So we need to size the gear for 100kA?, seems highly impractical and expensive for the customer.

    With regards to arc flash I explained to the engineer that only giving out max available is not neccesarily worse case for arc flash. He stated the fault current can vary. I said yes I understand and when we run the arc flash study we run it for several different fault currents but we would like to know expected fault current based on their system as it is today to assist in making sure we are choosing an appropriate set of conditions to calculate arc flash on.

    I stated I understand he was only following the direction he was given. I asked for a contact in his company to address this with. I would like to send a letter stating the concern to either get them to provide useful information or at least document I requested it and they denied. Anbody written a similar response to a utility that they might share?

    Any other thoughts on the situation and how you address it?
    Wilson99, Jun 4, 2010
    #6

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