Transformer explosion and its prevention is a complex industrial issue where various physical phenomena occur. To understand them, an experimental test campaign was performed by CEPEL on large scale transformers. It consisted in creating arcing in oil filled transformer tanks. The tests confirmed that the arc generates dynamic pressure waves that propagate in the oil. Reflections of these waves on the walls build up high static pressure to which transformer tanks can not withstand. This static pressure increase can be prevented by a quick oil evacuation triggered by the first dynamic pressure peak generated by the electrical arc. Moreover, a numerical tool was developed to simulate the phenomena highlighted during the tests and mainly the pressure wave propagation. It is thus based on a compressible two-phase flow modelling where viscous flow, electromagnetic, thermal and gravity effects are taken into account. The equations are solved using a finite volume method allowing computing complex 3D transformer geometries. Comparisons between the data obtained during the experiments and the results given by the simulations validated the model. It can be used to study transformer explosions as well as depressurisation induced by the explosion prevention technology based on a quick oil evacuation.