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| Utility provided system impedances https://brainfiller.com/arcflashforum/viewtopic.php?f=23&t=3716 |
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| Author: | DBL [ Wed Jun 03, 2015 12:29 pm ] |
| Post subject: | Utility provided system impedances |
Good Day, We were provided utility data, but it only included system impedances (R1,X1,R0,X0-PU on 100MVA Base). The utility company considers providing the fault current values a liability. What is the recommended method, convert to SC amps (reference Jim Phillips article, Making Sense of the Numbers, Utility Company Short-circuit Data For Arc Flash Studies) or input directly into software as ‘Zpu’ fault duty units, or other method. Will each method yield the same results? Thanks - DBL |
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| Author: | Jim Phillips (brainfiller) [ Wed Jun 03, 2015 6:05 pm ] |
| Post subject: | Re: Utility provided system impedances |
DBL wrote: Good Day, The utility company considers providing the fault current values a liability. That is a pretty interesting statement from the utility. The impedance data and the short circuit data are pretty much the same thing - just need a few calculations to go from one to the other. The article that you reference did not have a case with Z0. That is used when there is a ground/non-phase path so.... For a line-to-ground fault: I line-to-ground = (3xV) / (Z1 + Z2 + Z0) in your case (and in almost every case Z2 is the same as Z1) I line-to-ground = (3xV) / (2xZ1 + Z0) I show everything as a magnitude Z to make it simple. You have the better format of individual R and X values. The utility indirectly gave you the fault current. It is a direct function of the equivalent impedance which they gave you. Many utilities simply shroud this information in legal disclaimers and go ahead and provide it. |
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| Author: | DBL [ Mon Jun 08, 2015 4:10 am ] |
| Post subject: | Re: Utility provided system impedances |
Thanks Jim. To confirm, one can obtain Z with individual R and X values, using Z=SQRT[R squared + X squared]. |
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| Author: | jcp@pfeiffereng.com [ Mon Jun 08, 2015 7:04 am ] |
| Post subject: | Re: Utility provided system impedances |
For some programs you have to enter per unit impedance. I developed a spreadsheet to convert to Per Unit impedance. I tried to paste the spreadsheet but it was unreadable. I will be happy to provide it. |
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| Author: | engrick [ Mon Jun 08, 2015 7:13 am ] |
| Post subject: | Re: Utility provided system impedances |
I would rather have the impedance values, most of the time I only get a fault current. |
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