An example:
Plant rating 100 MVA and short circuit ratio=20: --> Short circuit power at POC = 2000 MVA.
X/R ratio = 20: --> Phi = atan(X/R) = atan(20) = 87.1376 degrees
RSORCE = cos(Phi) = 0.05
XSORCE = sin(Phi) = 0.99875
Assign a swing generator at POC bus with Mbase = 2000 MVA with RSORCE and XSORCE above. The absolute value of ZSORCE is 1 pu.
The generator will supply a short circuit power of Mbase/ZSORCE = 2000/1 = 2000 MVA with X/R ratio 20.
In dynamics, use model GENCLS with H=0 and D=0. This will represent a generator with infinite inertia and constant frequency (50 or 60 Hz).