# fault level

S

#### Santhosh

How does the synchronous impedance of a generator is calculated that is indeed used in fault calculations?

#### PhilCorso

Santhosh... a simultaneous 3-phase short circuit at or near an alternator’s terminals, can be represented by an Alternating-Current wave having a large value at fault initiation which then decays to a steady-state value some time later.

The three time periods normally assigned the transition period are: Subtransient; Transient; and Steady-state! The corresponding reactances are Xd", Xd', and Xs. Since Xs represents Synchronous Reactance then the other two are given as fractions of Xs.

Using a Turbo-alternator as an example, Xd", Xd', and Xs, could be 12%, 18%, and 160%, respectively!

Let us know if you need the complete fault-current equation in terms of the three reactances and their associated time-decay constants.

Regards, Phil Corso