Does pre-fault voltage affect short-circuit power?

[newengineer12]

Hi all

I have a question that bugged me for the last week. Does the pre-fault voltage of a bus in a power system affect short-circuit current & power in that bus? & Why?

 

[xnuke]

Yes. Ohm's law.

xnuke
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[bacon4life]

Voltage regulation via transformer load tap changers adjusting the transformer ratio will definitely change the short circuit value. Also, the impedance of a transformer may change as the windings are reconfigured by the LTC.

 

[waross]

And given the low power factor of faults the KVA will be many times the kW.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

Slide 29 shows how faults can be calculated with superposition and pre-fault voltages.

http://users.ece.utexas.edu/~baldick/classes/369/Lecture_18.ppt

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[Prot123]

Yes it does. this is the reason there is a guideline on voltage magnitude to be used during max/min fault calculation as per standards. If I remember correctly, for max fault 1.09V and min 0.91V in IEC.

 

[cranky108]

A better question might be, is the prefault voltage important.

If you want to maximum, then assume 1.09 PU voltage (or some normal high value).

If you want a motor start value, use a lower value.

 

[crshears]

Something worth remembering re this question is that power system operators have a very definite preference for keeping grid voltages at the higher end of the allowable range, as this generally optimizes power transfer capabilities.

Hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]