Measurement and FEM analysis of DC/GIC effects on transformer magnetization parameters

Topologically derived equivalent circuit models for transformers can be improved by more accurate parameterization of the magnetization characteristics. We examined the changes in transformer magnetization parameters by measuring the effects of dc on a scaled down model singlephase four-limb (1p4L) transformer and confirmed them with FEM modelling. The results define the non-linear character of the transformer inductance when partial saturation occurs, such as in over-excited transformers or with half-wave saturation caused by leakage dc or geomagnetically induced currents. The analysis clarifies the difference between the constant air inductance of a winding (L
_air
) and the instantaneously changing saturation inductance (L
_terminal
) used in low-frequency transformer equivalent circuit models and shows how to choose a representative value for the equivalent circuit parameter. The tests lead to a more accurate graphical depiction of related parameters during transformer half-cycle saturation than usually presented.