Numerical analysis of natural convection in a three-phase power transformer filled with industrial oil is considered. The paper presents a comparative analysis of the temperature and hydrodynamic fields when reducing a load factor by three different approaches to the simulation of the transformer tank area. The initial conditions of the velocity, pressure, and temperature fields are the results of calculating a stationary study of the convection in a power transformer. It is shown that considering the computational domain as a one-phase case significantly distorts the results. The appropriate way to simulate a cooling system is to use the inlet-outlet boundary conditions calibrated by physical experiments or numerical simulations. All the calculations were performed using the k - ϵ turbulence model implemented in Comsol Multiphysics. The presented results are of particular interest for the power transformer oil condition forecasting by numerical simulation tools.