Modelling of power transformers to predict and examine its performance in the presence of Geomagnetic Induced Currents (GICs) is of particular interest in research since these currents that are quasi-DC in nature drive the transformer into half-cycle saturation. During the half-cycle saturation, the increased leakage flux and harmonics have undesirable effects on the windings and the structural components of the transformer, which can possibly lead to their permanent damage. This prompts the need to perform a precautionary study of the transformer validating its robustness when subject to GICs. This thesis proposes a method to model the transformer and its electromagnetic (EM) behaviour under GICs that combines the accuracy of Finite Element Modelling (FEM) with the speed of Magnetic Equivalent Modelling (MEM) to produce a fast-computing yet detailed modelling method using simulation tools, COMSOL and MATLAB Simulink, respectively. The transformer under study in this thesis is a single-phase 280MVA, 500/230kV auto transformer designed by Royal Smit Transformers. It was first modelled using FEM in COMSOL, providing details of the equivalent 2D axisymmetric geometry of the core and the windings and the material properties to each component. The model was then computed and validated against factory measurements done by Royal SMIT for the transformer when not subjected to GICs. After which, the model was run for the condition in which the transformer is subject to a GIC. The main problem of failure of the model to converge before reaching steady-state condition under GIC was overcome after optimizing the solver settings of the FEM software. The difference in the EM behaviour of the transformer with and without GIC were studied. After ensuring that the FEM model was robust and accurately represents the transformer under GIC as well, the time-consuming part of computing the FEM model till it reached steady-state under GIC was then made to be taken care of by a MEM model developed in Simulink. Induced flux versus magnetizing current characteristics of the transformer were obtained from the FEM model, and fed to the MEM model, which only took a few seconds to run. The magnetizing current waveform obtained for a few cycles after reaching steady-state under GIC was then transferred to the FEM model, which could finally compute - for those values of magnetizing current - the detailed EM effects of GICs on the transformer, spatially. An important EM study needing the spatial distribution of the EM properties within the transformer is the calculation of eddy current losses in the windings. In this thesis, a method that only considers the envelope of the set of windings as opposed to modelling every strand to calculate their losses is developed using an analytical formula found in literature. This is validated against the losses calculated by Royal SMIT as well as the FEM software for a detailed model of the windings. The timesaving method to study the EM behaviour of transformers subject to GIC proposed in this thesis accurately models the transformer in the saturation region that should enable conducting EM studies furthermore to winding loss calculation with ease.