This paper aims to present the modeling and simulation of numerical differential protection of power transformers using the software ATP/ATPDraw and its MODELS environment. In order to validate the relay model, simulations were carried out using a simplified electrical power system whose parameters were obtained from a real system. The results reveal the advantages of using this type of modeling, since it provides a closed loop simulation, thereby one can evaluate the behavior of the power system against the protection operation, as well as the protection performance for the different power systems operational conditions.