A power system consists of a number of generating stations or infeeds which supply power at an acceptable voltage to a set of loads via a transmission or distribution network. The simplest network consists of series elements, each of which connects two nodes (junctions or busbars), and parallel elements which are considered to be attached to single nodes, possibly connecting them to neutral. The series elements are essentially transmission lines i.e. shunt susceptance, off-normal taps on transformers, shunt capacitors, loads and generators. It is assumed that only one load and one equivalent generator may exist at any node. For the purpose of analysis, transmission lines are represented by an equivalent network, with half the lumped susceptance inn each shunt arm. Transformers are represented by an equivalent network (see Appendix page). Loads are represented by their power demand and generators are represented by their power output at the machine terminals in the simplest case, as well as machine parameters in the stability program.