In this paper, a deep analysis of the behavior of transformer winding impedances for high frequencies is performed. The properties of inductance matrices are analyzed, as well as the possibility of making assumptions and their range of validity. The properties of resistance matrices are also analyzed, which has not been made in the past. The discussion on the influence of the iron core on inductance and resistance matrices is undertaken. The analysis is based on impedance measurements carried out on experimental transformer windings. Finally, recommendations are made about which are the most suitable impedance models for modeling transformers impedances during transient and resonance calculations.