Logic behind converting generator on Xd, X'd, X''d

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The CONG activity converts the generator using its subtransient reactance (Xʺd). In short-circuit studies, however, you can modify the generator reactance to calculate the fault current for the subtransient, transient, or steady-state intervals.

For example, consider a simple case with one bus and one generator with the following parameters:

                                  TYPE S X------POSITIVE SEQUENCE------X
   BUS#-SCT X-- NAME --X BASKV ID CODE T    R      X"      X'       X   
      1     GEN         132.00 1    3  1 0.00000 0.20000 0.50000 1.00000

The results from the short-circuit calculation (ASCC) are as follows:

 X------------ BUS ------------X         RE(I+)   IM(I+)
      1     [GEN         132.00] PU      0.0000  -5.0000    Using X''D Isc = (1.0 / 0.2)
      1     [GEN         132.00] PU      0.0000  -2.0000    Using X'D  Isc = (1.0 / 0.5)
      1     [GEN         132.00] PU      0.0000  -1.0000    Using XD   Isc = (1.0 / 1.0)

This shows how the choice of reactance (subtransient, transient, or synchronous) directly affects the calculated short-circuit current.