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 Post subject: Utilitys revisited
PostPosted: Mon Jul 12, 2010 4:40 am 
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Joined: Tue Feb 16, 2010 7:59 am
Posts: 21
Location: Minneapolis, MN
After numerous go arounds with a utility reluctant to provide available fault current data, and given our reluctance to storm the place with pitchforks and tourches, we were able to extract the following information:
The facility in question is fed from a substation with 3- 40 MVA transformers, "477 conductor" and nominal voltages of 139kV/12.4kV.
I am assuming that "477 conductor" means 477MCM and the conductor is almost certainly aluminum. We measured the transmission distance to the substation at 4.5 miles.
Now, a quick back of the envelop calculation using a table impedance value for 500MCM AL (wich is what I have handy) and guessing about 8% impedance for the transformer and 1% for the source, I am comming up with about 3800A which seems a fairly reasonable start.
My question is, Does anyone know what a reasonable impedance range for a 40MVA transformer might be.
Any input would be appreciated.

Bob Ragsdale, PE


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PostPosted: Wed Jul 28, 2010 8:26 am 
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Assuming the primary BIL is 550, ANSI C57.12.10 says 9.0% without load tapchanging and 9.5% with.

Note that with three transformers, there are three possible system configurations (source impedances) right there (neglecting all the other possible system changes the utility may make.)


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PostPosted: Wed Aug 11, 2010 3:52 pm 
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Joined: Mon Dec 14, 2009 4:24 pm
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I believe you mean C57.12.10


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PostPosted: Thu Aug 12, 2010 8:05 am 
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Fixed on edit. Thanks.


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