Generator running at reduced power? 2

[lz5pl]

I need to calculate short circuit current of a switchgear where co-generator unit will be connected in parallel with the grid. The specific point is that generator will be used at reduced power. Generator itself is 12 MVA, but the turbine will deliver only 8 MVA (limited by the available steam).
My question is: which value should I use for calculation of the reactance of the equivalent circuit - 12 MVA or 8 MVA.
According to me it should be 8 MVA, assuming that the generator will not be able to supply short circuit with energy equivalent to 12 MVA. But I am not absolutely sure, as my main field (control and relay protection) didn't help me to remember what exactly we studied 24 years ago in the University :~/.
Of course for dimensioning of switchgear panels I will use 12 MVA, but after adding the grid infeed we are going too close to the required 31.5 MVA (calculated is 30.9 MVA) and I just would like if we have some safety margin.

Any advice will be greatly appreciated!

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[00123456]

Hi,
Short circuit calculation should be based on unsaturated values X"d, X'd & Xd of machine w.r.t. T"do & T'd.

For example;

Machine MVA = 8MVA
KV = 11kV or 15.45kV
Base MVA = 10MVA
Base KV = 11kV
X"d = 18%
X'd = 24%
Xd = 250%

X"d in PU = (10*0.18)/(8) = 0.225PU
SC MVA = (10) / (0.225) = 44.44MVA
I"d = (44.44) / (1.7325*11) = 2.33kA for Inst. Tripping

X'd in PU = (10*0.24)/(8) = 0.3PU
SC MVA = (10) / (0.3) = 33.33MVA
I'd = (33.33) / (1.7325*11) = 1.74kA

Xd in PU = (10*2.5)/(8) = 3.125PU
SC MVA = (10) / (3.125) = 3.2MVA
I'd = (3.2) / (1.7325*11) = 0.167kA

Thanks

 

[lz5pl]

Thank you, 00123456. If I understand you correctly, I should use the real available power, not the rated one from the machine data.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[davidbeach]

No, the prime mover has nothing to do with fault currents. Fault current is strictly a function of machine itself.

 

[lz5pl]

Thank you, davidbeach, unfortunately that is true. I just asked also another friend, refreshed my university knowledge and came to the same conclusion :-(
I should look for other solution...

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant