Influence of Hydro Units´ Generator-to-Turbine Inertia Ratio on Damping of Subsynchronous Oscillations

The torsional data of hydro generators are examined and it is found that the inertia ratio between the generator and the turbine lies in the range of 10 to 40 and that the torsional frequency lies between 10 Hz and 24 Hz. It is shown that the high inherent generator to turbine inertia ratio of a hydro unit increases the modal damping at the torsional frequency significantly. This is substantiated by digital simulations for a generator connected to a series compensated AC network and by simulations on an HVDC-simulator for a generator feeding an HVDC-link. It is shown that with reasonable values of the turbine damping no torsional interactions problem would occur for the cases studied.