The increased complexity of power systems and related controls requires more and more sophisticated analysis tools. Moreover, actual systems are often managed close to their technical limits. Thus, the functions of security analysis, both static and dynamic, become mandatory. In particular, fast time domain simulations can be used for the dynamic security assessment.This paper presents an effective simulator to investigate in depth a wide variety of phenomena that can span from few tens of ms up to several tens of minutes. The classical time dichotomy among different dynamic phenomena is dealt with adopting an original technique based on an automatic switching from detailed models of components to simplified ones and vice versa according to the dynamic conditions of the system. The performances of the power system simulator will be pointed out by showing an application to a large size system.