Prediction of magnetizing current wave-forms in a three-phase power transformer under DC bias

A procedure based on the finite element method (FEM) is presented to determine the strongly distorted wave-forms of the magnetizing currents having given direct current (dc) components in each phase of a three-phase power transformer. The three phase voltages forming a symmetric sinusoidal system are assumed to be known. The relationships between the flux linkages of the three windings and the three values of the magnetizing current are determined using FEM and assuming static conditions. Suitable techniques are employed to establish the dependence of the dc components of the three magnetizing currents upon the three bias fluxes. The flux bias values yielding the given dc components are hence obtained by an iterative method yielding the wave-forms of the magnetizing currents having the prescribed dc values. The method is illustrated by a numerical example.