TORQUES IN MULTI-MACHINE GENERATOR SHAFTS DUE TO HVOC RECTIFIER RIPPLE CURRENTS SUPERIMPOSED ON THE DC CURRENT IN ASYNCHRONOUS LINKS

Abstract

ABSTRACT This paper analyses torsional vibrations in steam turbine-generator-exciter shafts of machines in multi-machine power systems in close proximity to the inverter station due to rectifier ripple currents superimposed on the DC current in asynchronous DC links. It extends earlier work to include an in-depth analysis of system scaling factors for modulation product harmonic currents impressed on generators not examined in the literature heretofore by an inverter in an asynchronous link. Frequency at which shaft torsional vibrations would be excited by modulation product harmonics in 50Hz/50Hz asynchronous links as a function of 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 steady resonant torque excitation is applied, the time constant for decay of the vibration and stiffness between adjacent cells is then discussed. The paper then shows that torque in shafts of machines in multi-machine networks may be estimated by proportioning HVDC Link inverter modulation product harmonic disturbance current appropriately to each machine at risk. A three phase fault studies programme to proportion converter harmonic current to each machine is discussed. System scaling factors are evaluated for different scenarios : (i) neglecting system loads, (ii) using a lumped load at the inverter, and (iii) employing distributed system loads for (a) constant impedance loads and (b) subtransient 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 in Ireland due to rectifier ripple currents superimposed on the DC current of the proposed North Wales/Ireland Link are then analysed and discussed.