Prediction of torsional oscillations in turbogenerator units connected to series compensated transmission systems

Electromechanical interaction between long-compensated transmission systems and turbogenerator units may produce a distinguishable mode of torsional oscillatory behaviour that does not occur with conventional systems. This interaction is a result of subsynchronous resonance between the compensating series capacitance and the system reactances. A serious situation can arise when the transmission circuits have a low value of resistance due to conductor bundling, in conjunction with a series resonant frequency which, when viewed in terms of individual rotor angular speeds, coincides with a natural torsional mode of a generator shaft system. The paper reports on a study of subsynchronous oscillations and their effect on the shaft systems of conventional and multiple shaft turbogenerator units. Emphasis is placed on the investigation of a practical range of mechanical-parameter variation encountered throughout the load range of a steam-turbine unit. The simulation model used in the numerous studies undertaken is summarised, and representative results are collected together to show the various conditions that give rise to this category of instability.