Abstract : The aim of this thesis is to study and compute eddy current losses in power transformers, in particular in their tanks, with the finite element method in 3D. The difficulties of the modelling of such devices concern : - small thicknesses of sheets, so that meshing them using volume finite elements lead to a large number of elements. - skin effects which can be pronounced. Mesh difficulties are increased, because of the great variations of the fields inside the skin. - non linearities of magnetic characteristic of materials. Taking into account these difficulties, we had to develop suitable numerical methods, which are : - several types of special elements, which allow to reduce the meshing problems. For static problems, these elements can describe thin gaps and thin ferromagnetic regions. For dynamic problems, these elements allow to model skin depth-independent thin conducting shells, bulk conductors where skin effects are pronounced, described with impedance boundary condition, thin weakly conducting regions and line conducting regions. - methods which take into account the saturation of magnetic materials in a harmonic regime (sinusoïdal). A first method, applied to volume elements, is based on an equivalence of energy and allow to compute global values only. Another method uses the impedance boundary condition calculated supposing the characteristic of the magnetic material is a step function. The special elements and the non linear methods have been validated comparing to axisymetric or 2D modellings. Simulation of a real transformer has been carried out so as to compute eddy current losses in its tank.