Modeling of the air-gaps of overlapped joints in three-phase transformer iron core for using by FEM

In this paper, it's established in 2D the distribution of the flux lines in joints of transformer core taking account of the industrial irregularity assembly. One use the model of an equivalent variable air-gap deduced from no-load measurement like magnetizing currents and flux curves. A new contribution to resolve the problem of the implantation of a variable air-gap length in programs based on finite element method FEM is highlighted. The new air-gap form is considered in an overall model of a three-phase transformer core. The new method resides in the change of the equivalent variable air-gap length (variable length according to flux level) to fixed one with the same magnetic drop. By taking this advantage on modeling in existing programs codes based on FEM, the real magnetizing currents are reproduced from the magnetic field distribution. Our approach is validated by comparison to experience results giving no-load current measurements for various saturations levels of a transformer 10kVA.