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 Post subject: Utility data-incorrect?
PostPosted: Fri Aug 12, 2011 4:03 pm 

Joined: Fri Aug 12, 2011 3:46 pm
Posts: 1
I was working on Short circuit study involving utility supply.
I confirmed with utility representative several time but All i received was following data...I do not trust this data, Please help me understand how can this be right data.

System Positive Sequence Impedance:
R= 7.41 ohms
X= 6.42 ohms

System Zero Sequence Impedance:
R0= 9.28 ohms
X0= 20.38 ohms

The X/R ratio in positive sequence is less than 1. Is it possible in any case.
I can not get any relaible dat after digging many times now.
Even in zero sequence case the ratio is hardly around 2.

the 3 ph Symmetrical short circuit duty is provided as 735 A
and single line to ground : 526 A
circuit base voltage: 12470 V

Thank you

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PostPosted: Mon Aug 15, 2011 11:44 am 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1676
Location: Scottsdale, Arizona
It certainly appears that something is incorrect. A low X/R is not that unusual if you have a low short circuit current since often it means you are at the end of a long small mostly resistive conductor.

However this is usually at low voltage. MV tends to be a bit more inductive. In any case, the short circuit current seems very very low. Is this on the end of a long line in a remote area? I wonder what kind of voltage drop they have based on such data.

It sounds like you should be asking them again to clarify the data.

Jim Phillips, P.E.

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PostPosted: Tue Aug 16, 2011 8:21 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 590
The voltage drop can be cured with regulators. Common construction in rural areas; run distribution lines long distances and insert a regulator when the voltage becomes unacceptable. For smaller conductor sizes (2/0 Al or #2 Cu), the resistance will exceed the reactance even at medium voltage.

Where is this installation, and how far away is the feeding substation?

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PostPosted: Wed Aug 17, 2011 11:55 am 

Joined: Tue Feb 16, 2010 7:59 am
Posts: 21
Location: Minneapolis, MN
While this seems very low, I encountered SC value of 980A on a 12.5 kV system. After some discussions with the utility rep, it turned out it was a case of too small a substation TR, with too high an impedance, located too far away, feed with too small conductor.
It would be a good idea to see if the utility rep can give you some of the parameters above. You can do a quick calculation to see if this is in the ball park.

Bob Ragsdale, P.E.

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