Investigation into the Impacts of Unbalanced GIC in Transformer Saturation Analysis

The part-cycle saturation of power transformers caused by the flow of Geomagnetically-Induced Currents (GIC) through power transformers is the root cause of the GIC issues which impact the bulk power system. One of the modeling assumptions made in Geomagnetic Disturbance (GMD) vulnerability assessments is that GIC is purely a common-mode signal. That is, it flows equally in all three phases of the power system, combining and returning through the neutral connection of a grid-connected wye transformers. This paper describes the impact of unbalanced GIC on grid-connected transformers. Field measurements were investigated to determine a realistic value of GIC current unbalance. The last section of this paper briefly discusses conclusions of the study and next steps.