ACTIVE DAMPING OF TORSIONAL MODES IN TURBINE-GENERATOR SHAFTS

Abstract − Subsynchronous resonance (SSR) may possibly occur in turbine-generators with long shafts and large inertias constituting a weakly damped mechanical resonator. Although SSR is most frequently encountered in power systems with series compensated transmission lines, electro-mechanical resonances can occur in all kinds of power sytems where synchronous generators supply critical loads. Any load causing active power fluctuations with frequency components in the same range as the mechanical resonant frequencies of the generator shaft assembly can be critical. Evidence for this statement is supplied by measurements which were performed with a 144 MVA and a 220 MVA flywheel generator of the IPP experimental power supply. Subsynchronous oscillations and resonances were caused in the shaft assemblies of these synchronous machines by thyristor converter loads supplying an experimental tokamak. The problems could be solved in applying a compact damper circuit which is connected in parallel to the dynamic loads. This novel damping method has been continuously applied at IPP in more than 3000 dynamic load sequences. Since it has proven to be very efficient and reliable, this paper presents results from numerical investigations on applying this active damping method as a countermeasure to SSR in large electrical networks. 1