According to VDE/IEC in standard system fed directly from generators without transformer units, the impedance defined in following equation has to be used.

Z_G = K_G x (R_G + j X_d")

With the correction factor

K_G =cmax/(1 + X_d" sin(phi))

Refer to IEC 60909 - 0 - 2001-07 Section 3.6.1 equation (17) and (18).

The reason for the difference is the impedance correction according to VDE/IEC standard.

For the case of system where the generator feeds a transformer, the generator with the transformer is automatically recognized as a "power station" by PowerFactory.

Here, in addition, a correction factor is also applied over the transformer according to equations

Z_kw=K_kw x (Z_G + Z_T)
K_kw=cmax/1 + (X_d^"-X_t^") x sin(phi)

Where X_t is the Short-Circuit voltage of the transformer, Z_G the generator impedance and Z_T the transformer impedance.