Impact of GICs on Power Transformers: Overheating is not the real issue

There 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.

Basics of the Effect of dc on Power Transformers

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 magnitude of the dc as well as the design of the transformer. The flux density shift adds to the ac flux in one half cycle and subtracts from it in the other half cycle, as shown in Figure 1(a). Figure 1.

The effect of dc on transformer core magnetization: (a) The flux density shift in the core caused by dc and (b) The part-cycle saturation of transformer cores.