Force calculation in transformer windings under unbalanced MMFs by a non-linear finite element code

The author considers the computation of the electromagnetic axial forces on the windings of a step-up-generator transformer under unbalanced MMFs of the same phase by means of a finite element code. The unbalance between the MMFs of the same phase, which might be on 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 MMFs is made by a nonlinear circuit model of a three-phase five-limb transformer. 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 the ratio between axial forces with and without MMF unbalance as a function of the impedance of the network with which the wrong parallel occurs