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3 phase Short Circuit value on the generator terminals

3 phase Short Circuit value on the generator terminals

3 phase Short Circuit value on the generator terminals

(OP)
Please help me with this one.
I tried to prove results of the software we use in my company for simple radial generator system. I introduced fault at the end of the generator (made generator as Swing Source). My calculation on the paper yields 3 phase fault current result of 9.014 kA. When I ran software model with same generator data (rated 7500kVA, terminal voltage 4.16 L-N, Xg’’=20% ) it yields result 3.124 kA. I wonder why?
I've figured that my calculation should have been done with 2500kVA (which is per phase KVA rating of the generator) instead of 7500 kVA.

This is a reply from software Consultant:
(quote:)
"...If the 4.16 kV is the line to ground voltage, then the base impedance for the machine is 4.16*4.16/2.5 = 6.922 Ohm.  If Xd" is .2 pu, then the impedance is .2 x 6.922 Ohm = 1.384.  Then the fault current for a 3 phase fault at the machine terminals is
If = 4.16/1.384 = 3.0048 kA.
I set up the case in Adept, using a synchronous machine for the source, 7500 kVA, Xd" = .2 pu and got 3.0047 kA as the result. "
(end of quoting)

I' ve been thought to use 3 phase SC rating in the equations. Can I have somebody to verify this result on different software maybe? Or explain why am I wrong?

Thank you for your input.

RE: 3 phase Short Circuit value on the generator terminals

You can use either single phase or three phase quantities, but you have to be consistent.  If you use single phase kVA, then use phase-to-neutral voltage.  If you use three phase kVA, then use phase-to-phase voltage.

The equation for base impedance is the same (kV²/MVA), but the equation for base current is different.  kVA/kV using single phase quantities and kVA/[sqrt(3)*kV] using three phase quantities.

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