Dynamic simulation of synchronous machines interconnected by long, compensated transmission circuits

Systems incorporating long, compensated transmission lines operating in conjunction with salient-pole hydro-generators, necessary in the full utilisation of hydropower in remote regions, may operate in particular modes that do not occur with the more conventional power systems. When series-capacitor compensation is used, the line natural frequencies are modified and an extra oscillatory mode is introduced at a frequency which may interact with the machines in the system and cause instability. The paper reports on a study of transmission-system initiated oscillations at subsynchronous frequency, brought about by the introduction of series compensation, and self-induced oscillations at a frequency coinciding with the electromechanical natural frequency of the individual machines. A mathematical model of the complete system is developed that is suitable for use in computer studies. Representative results, from the numerous studies based on this model, are collected together in the paper, and are used to show the conditions which give rise to the two categories of instability.