Geomagnetically Induced Currents (GIC), flowing in power systems during Solar Magnetic Disturbances (SMD), can cause severe offset saturation of power system transformers. Continuous saturation of transformers can result in transformer overheating, and cause harmon c currents to be injected into the system. Relay and protection systems are affected by harmonic currents passing through the system. A considerable amount of study has been done using analog and/or digital simulations of transformer half-cycle saturation to predict the transformer response to DC excitation. These methods used simplified transformer core models to represent the excitation-flux relationships. This thesis includes the development of an accurate transformer core model. The accuracy of the model is validated by comparing the recorded waveforms during GIC events with the simulated waveforms using the model. Then the model is used to investigate the other system quantities at different GIC conditions. As well, the performance of several protection schemes under GIC are evaluated. The study is performed using an electromagnetic transients simulation program.