STAG-a new unified software program for the study of the dynamic behaviour of electrical power systems. A program is developed for the numerical simulation of dynamic phenomena in power systems. This program unifies the conventional fields of transient, mid-term and long-term stabilities. The integration algorithm ensures a constant level of accuracy in the calculation of fast or slow transients through a continually variable step and through suitable numerical stability properties. The performance levels reached, both as regards ease of utilization and the size and complexity of the simulated systems, allow the majority of cases to be covered so far as the dynamic system behavior study requirements of the Belgian power companies are concerned.