Abstract : The knowledge and the control of many transient states which appear and propagate on the networks and in the materials which are connected there (transformer, lightning protectors,...) is a current problem. A transformer of power connected to a Medium Average network or very high voltage is subjected to various types of overpressure whose knowledge makes it possible to evaluate the constraints to which are subjected the internal insulations. On the one hand, we worked out a new model of valid transformer shell on a very broad frequency band, resulting from the principle of the electric magnetic duality in order to know its internal behavior (frequencies of resonance, over-tension coefficients). This model is intended to be introduced into computer codes of networks, for example EMTP. The model takes into account the losses in copper and the magnetic core. Methods were also developed for a reliable calculation of the parameters of the model. In addition, a method of modal analysis making it possible to illustrate the oscillatory behavior of a transformer was developed. This method leads to an equivalent diagram, which is used to study the interactions of the transformer with the network. In these preceding models, the calculated results of simulation are in concord with those obtained by measurement, as well on small-scale model as on real transformer VHV. In addition, a calculation algorithm of electrical supply networks founded on I' inversion of the transform of Laplace was developed. II the inversion of the transform of Laplace of several functions allows at the same time, by adapting the nodal method to it. The computation results are in very good agreement with those of measurements and make it possible to hope for a better simulation of the system line-transformer.