TORQUES IN MULTIMACHINE GENERATOR SHAFTS DUE TO HVDC RECTIFIER RIPPLECURRENTS SUPERIMPOSED ON THE DC CURRENT IN ASYNCHRONOUS LINKS

This paper analyses torsional vibrations in steam turbine-generator-ex citer shafts of machines in multi-machine power systems in close proxi mity to the inverter station due to rectifier ripple currents superimp osed on the DC current in asynchronous DC links. It extends earlier wo rk to include an in-depth analysis of system scaling factors for modul ation product harmonic currents impressed on generators not examined i n the literature heretofore by an inverter in an asynchronous link. Fr equency at which shaft torsional vibrations would be excited by modula tion product harmonics in 50Hz/50Hz asynchronous links as a function o f deviation in system frequency is first reviewed. Amplitude of shaft torque due to steady resonant torque excitation which is a function of initial rate of increase of vibration at adjacent cells where the ste ady resonant torque excitation is applied, the time constant for decay of the vibration and stiffness between adjacent cells is then discuss ed. The paper then shows that torque in shafts of machines in multi-ma chine networks may be estimated by proportioning HVDC Link inverter mo dulation product harmonic disturbance current appropriately to each ma chine at risk. A three phase fault studies programme to proportion con verter harmonic current to each machine is discussed. System scaling f actors are evaluated for different scenarios : (i) neglecting system l oads, (ii) using a lumped load at the inverter, and (iii) employing di stributed system loads for (a) constant impedance loads and (b) subtra nsient reactance dynamic loads. The effect of tappings of transformers is also discussed. On the basis of these evaluations, generators are identified which are most at risk. Torques in shafts of the machines i n Ireland due to rectifier ripple currents superimposed on the DC curr ent of the proposed North Wales/Ireland Link are then analysed and dis cussed.