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| Utility data-incorrect? https://brainfiller.com/arcflashforum/viewtopic.php?f=4&t=1810 |
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| Author: | kunasalu [ Fri Aug 12, 2011 4:03 pm ] |
| Post subject: | Utility data-incorrect? |
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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| Author: | Jim Phillips (brainfiller) [ Mon Aug 15, 2011 11:44 am ] |
| Post subject: | |
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. |
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| Author: | stevenal [ Tue Aug 16, 2011 8:21 am ] |
| Post subject: | |
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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| Author: | BCR_1962 [ Wed Aug 17, 2011 11:55 am ] |
| Post subject: | |
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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