Importance of core joints in GIC/dc studies with scaled down laboratory transformers

The response of large power transformers to geomagnetically induced currents (GICs) cannot be tested fully in factories because of the significant reactive power required. While carrying out physical tests on scaled-down models of a power transformer, we discovered that models with high-grade electrical core steel and even a transformer tank, as widely used in investigating the bias effects of GICs and leakage dc, cannot replicate adequately the actual performance if they are built without relevant construction details. Laboratory testing and FEM simulation of three single-phase four-limb transformers show that transformer models need multi-step lap mitered joints similar to those in full-scale power transformers to reproduce correctly the saturation response to dc components. FEM simulation also shows the change of transformer inductance when driven into deep saturation, as required in equivalent circuit models. The approach of achieving consistency between equivalent circuit, FEM and physical models even in the presence of saturable core joints is a novel contribution which provides guidance for future research in slow transient phenomenon studies.