The main purpose of this paper is to provide a methodology to monitor harmonics produced by power transformers during geomagnetic disturbances. The advocated scheme leverages the existing infrastructure and is based on the embedded operating quantities of microprocessor-based differential protection relays. The characteristics of single-phase transformer magnetizing current are discussed. The second harmonic signature is a good indicator of half-cycle saturation and can be mapped into an effective geomagnetically induced current. The proposed scheme was tested in closed-loop using a real-time digital simulator and validated with field measurements.