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

Z_GK = K_G x Z_G = K_G x (R_G + j X_d")

With the correction factor

K_G = Un/UrS * cmax/(1 + X_d" √(1-cos^2(phi) ) )

where:

cmax: voltage factor according to Table 1 (Section 5.3.1)

X_d": saturated subtransient reactance of the generator

cos(phi): power factor of the generator under rated conditions

Un: nominal system voltage

UrS: rated voltage of synchronous generator

Refer to IEC 60909 - 0 - 2026 Section 6.8.2 equation (31) and (32).

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.

Therefore for the calculation of short-circuit currents of power station units the following equation for the impedance of the whole power station unit is used for short circuits:

Z_kw = K_kw x (tr^2*Z_G + Z_T)
K_kw = cmax /tr^2 * 1 / (1 + | X_d" - X_t | x √(1-cos^2(phi) ))

Where

X_t: relative reactance of transformer,

Z_G: generator impedance (without correction factor K_G)

Z_T: transformer impedance (without correction factor K_T)

tr: rated transformation ratio of the step-up transformer

Refer to IEC 60909 - 0 - 2026 Section 6.11.2 equation (41) and (42).