FORCE CALCULATION IN TRANSFORMER WINDINGS UNDER UNBALANCED MMFS BY A NONLINEAR FINITE-ELEMENT CODE

This paper deals with the computation of the electromagnetic axial forces on the windings of a step-up-generator transformer under unbalanced MMF of the same phase, by means of a finite element code. The unbalance between the MMF of the same phase, which might be of the order of the nominal MMF, is caused by the high saturation of the iron core, which occurs during the transient due to a wrong parallel of the transformer with a network having a very low impedance. The evaluation of these MMF is made by a non linear circuital model of the three-phase five-limb transformer[1]. With the peak values of MMFs, the magnetic field in the transformer window is computed by a two-dimensional nonlinear code, and then, the Lorentz forces on the windings are evaluated. The result of the computation is expressed as ratio between axial forces with and without MMF unbalance, as a function of the impedance of the network with which the wrong parallel occurs.