Characteristics of transformer exciting-current during geomagnetic disturbances

An efficient method allowing the transformer flux offset and exciting-current waveform to be determined for combined AC and geomagnetically induced current (GIC) excitation, without lengthy time-domain simulations, is described. This iterative approach solves for the equilibrium saturation condition using lumped-parameter nonlinear magnetic-circuit models. In addition to investigating single-phase transformer performance, the authors examine the performance of certain three-phase transformer types for which GIC results in phase imbalance