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 Post subject: Utility supplied fault calculations
PostPosted: Sat Feb 20, 2010 10:31 am 

Joined: Thu Apr 02, 2009 4:33 pm
Posts: 29
Location: OH
I would like some opinions on fault current. The utility supplied a fault current calculation of 37269 amps on the secondary side of a 300 kva pad mount trans., 12470/208 volt 3 phase wye to wye transformer with %R 402 + %jX 627 on a 100 MVA base.
If I calculate the fault current using the nameplate impedance of 2.95 %, I come up with 28,228 on the secondary at infinite bus. It is very unlikely that this transformer will ever be replaced unless a failure occurs, and I have yet to see one replaced for failure where I am at (I've seen a lot of pole mounted cans replaced but never a pad mount). That is quite a difference in amperage in my opinion between my calculation and the utility. Which one would you use?


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PostPosted: Sat Feb 20, 2010 1:01 pm 

Joined: Mon Feb 15, 2010 6:40 pm
Posts: 2
Utility supplied. they set the fault currents suppling a service. weather they are right or wrong.
But you can call the utility designer/engineer and go over the calculation. if you feel its high.


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PostPosted: Sat Feb 20, 2010 1:44 pm 

Joined: Thu Apr 02, 2009 4:33 pm
Posts: 29
Location: OH
Correct me if I am wrong. In the above problem, are my calculaions right using the transformer data plate impedance? And if right, is that the maximum fault that could be delivered by the utility? That is excluding motor contribution. The utility already has a disclaimer that their calculation could be more than 37269 amps with nearby motor contribution. I am to believe the utility did not figure the 2.95% impedance in their calculation and based it on something else?
Also, I do not know how everyone else's utility is, but mine takes weeks and sometimes months for information and replies. Like everyone else, they are doing more with less.


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PostPosted: Mon Feb 22, 2010 12:05 pm 
Sparks Level

Joined: Wed Jan 28, 2009 2:19 pm
Posts: 72
Location: Georgia
I would continue to press the utility for actual numbers, but if they are not cooperating, I would run different scenarios. Start with your calculated infinite bus. Then run another scenario using 70% of the infinite bus. Then another scenario using 40% of the infinite bus. Use the results from your worst case for your report and labels. Some buses may be worse case at 40%, some may be worse at infinite bus...but you should be covered.

Other people may use other percentages. I'd be curious as to what others may recommend.


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PostPosted: Mon Feb 22, 2010 12:23 pm 

Joined: Thu Apr 02, 2009 4:33 pm
Posts: 29
Location: OH
Thanks, JJH. I'll try that


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PostPosted: Wed Feb 24, 2010 1:11 pm 

Joined: Thu Apr 02, 2009 4:33 pm
Posts: 29
Location: OH
Just an update for everyone. I received a call from the utility engineer today. I was able to get the impedence of the transformer that they used for their calculations. The utility used a 2% IMP., even though the data plate says 2.95%. By using the 2% IMP I was able to calculate the source MVA as 128 using the rest of the utility supplied information. Thanks for all the replies.


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PostPosted: Thu Feb 25, 2010 11:56 am 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 498
Location: New England
That still seems high to me unless its a very beefy line or a tap for a intrastate feeder. Try also using 50MVA as a low value and see what it does to your calcs.


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