Power links with Ireland-excitation of turbine-generator shaft torsional vibrations by variable frequency currents superimposed on DC currents in asynchronous HVDC links

The paper describes an in-depth analysis of excitation of shaft torsional vibrations in steam-turbine-generator-exciter shafts in close proximity to HVDC converter stations by variable-frequency ripple currents superimposed on the DC currents in asynchronous links. It extends earlier work to include an in depth analysis of system scaling factors for harmonic currents impressed on generators in Northern Ireland by an inverter and to investigate the phenomena for possible torsional vibrations in the generators by the link. It is concluded that variable-frequency ripple currents superimposed on the DC current in asynchronous links can excite sympathetic torsional vibrations in turbine-generator-exciter shafts. Subtransient generator reactances may be used to approximately proportion injected harmonic current to each machine of a multi-machine network. Very small noncharacteristic currents could result in serious damage to the machine although possibility of two power systems operating with fixed deviation in system frequencies long enough for the vibrations to build up is remote. Studies should be undertaken as routine on all machines in close proximity to HVDC converter stations to ascertain whether or not the machines are at risk