The short-circuit currents are calculated with the equivalent voltage source at the fault location. For an explanation of the theory behind short-circuit calculations according to IEC 60909 please refer to the norm or secondary literature:
IEC 60909-0:2016 Short-circuit currents in three-phase a.c. systems
pandapower currently implements symmetrical and two-phase faults. One phase faults and two-phase faults with earthing are not yet available.