Modeling transformer operations under dc bias

The ever-growing number of publications on transformer modeling under saturation conditions, for instance during geomagnetically induced currents and inrush current events, shows that this problem remains far from being solved. This work is a follow-up investigation directed at improving topological models of three- and five-legged core-type transformers operating under abnormal conditions of high flux densities in the core. The difference in behaviors of these transformers under saturation is explained by different reluctances they impose on the zero-sequence fluxes. The modeling of two 3-phase 3-legged 50kVA transformers is based on terminal voltages and currents measured in a special-purpose saturation test. The terminal loops are recalculated into effective loops of the gapped core material taking into account a typical saturation curve of grain-oriented electrical steel. The loops obtained are then used in individual hysteretic inductors of the legs and yokes employed in transformer topological models. When modeling the unique Fingrid experiment with two 5-legged 400 MVA transformers, it was shown that the positive- and zero-sequence impedances of the supplying network have to be taken into account. For the accurate modeling of the three-leg transformers, the measured values of individual dc phase voltages had to be evaluated and introduced in the model. The accuracy achieved in modeling terminal currents allows the developed models to be used to study magnetization processes in the legs and yokes of the core. The approach represented in the chapter provides a basis for modeling transformer transients for a wide range of abnormal conditions in power systems.