[en] Geo-magnetically induced DC currents (GICs) due to solar activities may yield strong DC magnetization of transformer cores, as a source of over-heating and even of destruction. The paper shows that distinct planar eddy currents (PECs) are generated by stray flux, mainly due to strong increases of excitation currents. The amount of effects rises with increasing the DC back-flux admittance. The latter proves to be a minimum for three-phase operation without tank, and a maximum for one-phase operation. Local PEC losses can be determined in a direct way through the detection of eddy current field components by means of thinnest gold films