Transformer Impedance Correction Factor
IEC 60909 uses a simplifying assumption where the pre-fault voltage at the fault location is not calculated (e.g. by a load flow), but rather an equivalent voltage source is used (assumed to be the nominal voltage multiplied by some voltage factor c). Moreover, two further issues complicate the matter: 1) the pre-fault load current is not known (i.e. not calculated in the IEC 60909 method) and 2) transformers can operate at a wide range of off-nominal tap settings. As a result, the simplified IEC calculations do not align with calculations based on the more accurate superposition method. An impedance correction factor was thus introduced in IEC 60909:2001 to improve the accuracy of the calculated fault currents.
In the older versions of IEC 60909 (pre-2001), there was no explicit impedance correction factor for transformers, instead just a suggestion that considerations need to be made. IEC found this unsatisfactory and introduced the correction factor in the 2001 edition. The correction factor was essentially defined empirically through a statistical analysis of a few hundred transformers. Most of this is described in some detail in IEC 60909-1.