Reconciling Tap-Changing Transformer Models

The model of the tap changing transformer used in classic power system studies, including load flow analysis or state estimation, is still somehow controversial. Two alternative formulations can be found in the literature, which have been adopted by the most important software packages. This work demonstrates that those formulations lead to similar results near the principal tap but to important discrepancies at extreme tap positions, with different impact depending on the power factor of the power flowing through the transformer. Moreover, a general model that fully explains those differences is proposed. The new model allows to adopt a third alternative that, without requiring further data than those used by traditional formulations, leads to highly improved results.