Subtransient Reactance of Generator

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"subtransient reactaNce"

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I guess 12.5% you mentioned is the Xd', check the name plate!

X'd is not the same as Xd, this is engineering science, not political science, guessing is inappropriate.

You can't. This is not an exercise for handbook engineers, nobody can tell you except the manufacturer.You could use guesstimates (see above), but since X"d is a critical parameter in stability and short circuit studies, an error in your "guess" could result in a false sense of stability and/or undersized protective equipment.

A value of 12.5% of transient reactance for rating of alternator seems low, until the alternator was a special ordered oversized alternator for applications such as too much non-linear loads or to cope up with voltage dip while starting a motor.

Otherwise I am not aware of any calculations, Xd" and Xd' are determined by testing during shop tests (sudden short circuit test say at 50% of rated voltage).

In case Xd" is not available a good guess can be 50% of Xd'. My experience with 100s of small 3 phase salient pole synchronous alternators is that Xd" is approximately 50% of Xd'. This shall be a conservative figure for maximum fault or peak fault current calculations, as even IEC permits a Tolerance of +/-20% in these parameters.

Thank u all for the replies.I understood that subtransient reactance of generator will be mentioned by manufacturer.

The synchronous generator reactance of 12.5% could be X"d [subtransient reactance] but only if it is a salient poles machine. See ABB Switchgear Manual 11 ed. ch. 3 Calculation of Short-Circuit Currents in Three- Phase Systems. Table 3-6 Reactance of synchronous machines.