This paper describes the theoretical basis and application of a simulation package for electrical generation, transmission and distribution systems. The software provides an environment for the development and testing of online analysis and control algorithms, and also has potential for application in operator training. Dynamic and algebraic models of power system elements are described which are appropriate for study time scales of up to one day. Numerical techniques are presented which obtain solutions with acceptable accuracy in real time for medium sized networks using minicomputer hardware. Typical results are given for a test network of 30 substations which illustrate the dynamic behaviour of the network under emergency conditions