3 phase Short Circuit value on the generator terminals

[pajce]

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.

 

[jghrist]

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.