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 Post subject: Pre-fault Voltage
PostPosted: Thu Sep 26, 2019 10:22 am 

Joined: Thu Jun 09, 2016 6:17 pm
Posts: 6
Hello,

When conducting fault current calculations prior to deriving incident energy values, does anyone run multiple scenarios with different pre-fault per unit voltage values?

Thanks for the input.


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 Post subject: Re: Pre-fault Voltage
PostPosted: Sat Sep 28, 2019 4:00 am 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1509
Location: Scottsdale, Arizona
Back in my electric utility days we would run V at 1.05 p.u. as a what if in the event transformer taps boost the voltage by 5% I don't if too many others take this approach. It really becomes important when the fault current is close to the interrupting or withstand rating.


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 Post subject: Re: Pre-fault Voltage
PostPosted: Mon Sep 30, 2019 7:49 am 

Joined: Mon Nov 10, 2014 11:01 am
Posts: 22
I normally run three scenarios; Base analysis, utility drop by 10%, and all motors off. The 10% voltage drop (0.9 utility voltage) is to take care of a heavily loaded utility system.


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 Post subject: Re: Pre-fault Voltage
PostPosted: Mon Sep 30, 2019 10:46 am 
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Joined: Tue Oct 26, 2010 5:00 pm
Posts: 188
Location: Maple Valley, WA.
We typically do not change the prefault voltage. Instead we run multiple scenarios. (1) Infinite Utility fault current (when primary fault current is not known) with motors on; (2) Double the Utility transformer Impedance with motors off. If the system has emergency generators, then we run additional scenarios with (3) Generator On - Motors on and (4) Generator On with motors off.

This will produce the maximum and minimum fault currents for each location.

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Robert Fuhr, P.E.; P.Eng.
PowerStudies


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