For the purpose of calculation and studying of steady-state asymmetrical modes of operation of three-windings, three-leg power transformer with an arbitrary winding connection scheme the mathematical model is proposed. This model makes it possible to take into account electromagnetic coupling between the windings, located on different legs. Windings connection scheme in developed model can be arbitrary, including some more complex schemes such as “double zigzag”, “polygon”, “triangle with extended sides”, etc. As the initial data for the model elaboration it is used usual datasheet for power transformers, including nominal values of power and voltages of the windings, short circuit voltages of the winding pairs, power losses in no-load and short circuit modes, no-load current, as well as additional similar data on transformer zero sequence parameters, provided additionally by manufacturer. Use of zero-sequence data allows reflecting one of the most important features of a three-leg transformer, in comparison with three-phase group of single-phase transformers due to electromagnetic coupling between windings, located on different legs. This leads to some peculiarities of its asymmetric modes. Calculations and analysis of the usual steady-state asymmetric modes, including short circuits and phase breaks were carried out on the base of power transformer with windings connection scheme Yn/yn/d-11. The conditions were found out when the magnetic flux tends to exit the magnetic circuit, which can lead to additional losses and local overheating. For considered modes, vector diagrams of currents and voltages of the windings, as well as relative values of magnetic fluxes in the legs are constructed.