By Ramsis S. Girgis and Kiran B. Vedante Impact of GICs on Power Transformers Overheating is not the real issue. HERE HAS BEEN SOME MISCONception in the electric power industry that geomagnetically induced currents (GICs) can cause significant overheating damage to the majority of medium and large power transformers. Because of the nature of GICs, the majority of power transformers would not experience damaging overheating because of them. This article presents the real issue with GICs, which is that the combination of the increase of reactive power absorption and injected current harmonics into the power system could result in compromised grid stability. To understand the effect of GICs on power transformers, this article starts by presenting the phenomenon of part-cycle saturation of transformer cores due to dc and the resulting magnetizing current pulse. This is followed by a description of the increase in transformer windings and structural parts' temperature rises associated with dc. Calculated values of these temperatures, considering the nature of the GIC pulses in light of their short durations, are then presented. T basics of the effect of dc on Power Transformers Phenomenon of Part-Cycle Saturation of Transformer Cores When Subjected to dc When a power transformer is subjected to dc, the result is a unidirectional dc flux in the core. The magnitude of the flux depends on the Digital Object Identifier 10.1109/MELE.2015.2480355 Date of publication: 2 December 2015 8 I E E E E l e c t r i f i cati o n M agaz ine / december 2015 2325-5987/15©2015IEEE