The present trends in power-system design cause a continual reduction of the stability margins of generators connected to the British Grid system. This is due, primarily, to the changing characteristics of the turbo-generators, and, for this reason, the need has arisen for an accurate generator model for use in stability studies of future systems.The paper describes the development of a mathematical model which includes many of the features normally neglected in conventional network-analyser and digital-computer studies. The model includes mechanical and electrical damping, flux variations, iron saturation and saliency, and it also permits the inclusion of voltage-regulator and governor action. By solving the equations on an analogue computer, the accuracy of the mathematical model is assessed by comparison with test results obtained on a 30 MW turbo-generator. Recommendations are then made for improving the model.