TORQUES IN TURBINE-GENERATOR-EXCITER SHAFTS DUE TO DC CURRENTS IN ASYNCHRONOUS HVDC LINKS

This paper makes an indepth analysis of excitation of shaft torsional vibrations in steam turbine-generator-exciter shafts by variable-frequ ency ripple currents superimposed on DC currents in asynchronous Links . First, frequencies at which sympathetic shaft torsional vibrations w ould be excited by modulation product harmonics in 50Hz/50Hz asynchron ous Links as a function of deviation in system frequency is illustrate d. Then steady-state torque excitation due to injected harmonic stator currents due to the DC ripple currents is analysed. Time constants fo r decay of shaft torsional vibrations based on electrical and steam vi scous damping for generators operating at full-load and no-load for re duced shaft models which are used with the torque excitation to give a mplitude of modal torque are also evaluated. Amplitude of shaft torque for a given torque excitation using reduced shaft models is illustate d. The effect of generator load on shaft torque which results from res onant excitation for reduced shaft models is also illustrated. Typical shaft torques for generators connected close to HVDC Link inverters w hich results from resonant excitation by non-characteristic currents a t full-load and no-load are also illustrated. Shaft torques in multi-m achine networks are estimated by proportioning HVDC Link disturbance c urrent to each machine at risk using system network data, generator da ta, fault analysis data, and load flow data, considering the frequency dependence of the system parameters. 660MW and 1000MW machines are co nsidered in the analyses that are made.