Why are the results for Short Circuit calculation different depending on "Power Station Unit Detection" option?
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_GK = K_G x 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.
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 (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 (uk),
Z_G the generator impedance (without correction factor K_G)
and Z_T the transformer impedance (without correction factor K_T)
Refer to IEC 60909 - 0 - 2001-07 Section 3.7.1 equation (21) and (22).