The flow of GIC in transformers is the root cause of all GMD related power system problems.
The magnitude of the GIC current and the associated DC offset is superimposed on the excitation current which forces the transformer into part cycle saturation.
GIC can result in increased reactive power requirements and large harmonic currents during GMD events.
The harmonics can result in tripping of VAR compensation devices at times when additional VARS are needed most, resulting in system disturbances and instability. Large GMD events are often associated with a variety of system alarms.