In this paper, the investigation is focused on the topology-based geometric models of power transformers. These models take into account any type of core design, the geometrical characteristics of the magnetic core, and the nonlinear characteristics of the core material. These models differ from each other in the way in which the magnetic part is described. Themethods, which have been proposed for the description of the magnetic part of a power transformer, are based on the conventional magnetic circuit of the core and present disadvantages. In this paper, a new method for the derivation of the differential equations of the magnetic core of the transformer is proposed by introducing a newauxiliary incremental circuit, more easily handled mathematically in antithesis to the conventional magnetic circuit that is used in the literature until now. Specifically, using the proposed circuit one can directly write the differential equations of the magnetic part of the transformer, the coupling between the windings of the transformer is easily calculated, the equations of the electric and the magnetic part are decoupled as well as the state equations of the electrical part can be written in standard form. The proposed method can be combined with anymethodology presented in the literature on the core losses modeling without any restriction. The simulation results show forth that the proposed method of transformer modeling is suitable for transients and power quality studies. Moreover, the proposed method can be a useful tool on sensitivity analysis during the design stage of small and large power transformers as well. © Springer-Verlag 2012.