I. Introduction
Geomagnetically induced currents (GIC) are quasi-dc currents with frequencies that range between 0.001~0.1 Hz [1] . GICs are not the sole source of dc currents in power grids. DC current may be injected from HVDC lines operating near ac grids, photovoltaic systems connected to the grid via inverters, and ac and dc drives. Their effects on the power system are costly to power utilities. In South Africa, several transformer were damaged during the Halloween Storm [2] . The primary effect of GIC current to transformers is asymmetrical saturation. Asymmetrical saturation lead to increased harmonics, noise, magnetization current, voltage drop, hot spot temperature rise and reactive power demand. Large excitation currents flowing in the power system under dc-bias have also been found to cause false tripping of relays especially in capacitor banks and static var compensators. Tripping of these reactive power reserves may lead to catastrophic events such as blackouts or even permanent damage to insulators, capacitors, static var compensator (SVC) firing thyristors and transformers. Mal-operation of relays is also linked to even and odd harmonics generated by saturated transformers conducting GIC. The main focus of this paper thus dwells on the harmonic content of transformers under GIC attack, and the levels of dc current causing such damages. High magnitudes of magnetizing current that is rich in harmonics results in high flux in the core and its surroundings. The increase in flux results in increased core losses and hotspot temperature within structural parts of the transformer. This additional heating causes insulation degradation that may cause premature failure of the transformer.