D.C. Component in Transformer: Physical Behaviour and Design Features

The behavior of a ferromagnetic core is analyzed, when the windings (wound on the core) are contemporarily fed by a sinusoidal voltage and a direct current. After the theoretical analysis of the problem, an analytical/ numerical study is carried out on a mono-phase transformer, using a simple circuital model, which takes into account iron non linearity; two typical cases are considered, of a mono-phase transformer which feeds a converter (d.c. current is in secondary winding) and of a transformer fed by a converter (d.c. current in primary winding). Finally, some suggestions are given about two design features, very important in presence of a d.c. component: the use of additional air gap and, for a three phase transformer, the type of winding connection (wye or delta).