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 torsio nal vibrations in steam-turbine-generator-exciter shafts in close prox imity to HVDC converter stations by variable-frequency ripple currents superimposed on the DC currents in asynchronous Links. It extends ear lier work to include an indepth analysis of system scaling factors for harmonic currents impressed on generators in Northern Ireland by an i nverter and to investigate the phenomena for possible torsional vibrat ions in the generators by the Link. Frequencies at which shaft torsion al vibrations would be excited by modulation product harmonics in 50Hz /50Hz asynchronous Links as a function of deviation in system frequenc y is reviewed. Amplitude of shaft torque due to steady resonant torque excitation which is a function of initial rate of increase of vibrati ons al adjacent cells where the steady resonant torque excitation is a pplied, the time constant for decay of the vibration, and stiffness be tween adjacent cells is discussed. Generator scaling factors for derai led (2d,3q) and reduced rotor circuit models which relate rate of torq ue excitation to harmonic current impressed on the generator stator ev aluated by different methods are also discussed. The impedance of the generator to harmonic current which can excite turbine-generator-excit er shaft torsional vibrations is then reviewed. Relative noncharacteri stic current levels for 50Hz/50Hz connectors are illustrated assuming ripple currents at the inverter which gives realistic harmonic voltage s in a twelve-pulse bridge. The paper then shows that torques in machi nes in multimachine networks may be estimated by proportioning HVDC li nk harmonic disturbance current appropriately to each machine at risk. Here, the impedance of each generator, transmission line, transformer and connected load at subsynchronous and supersynchronous harmonic fr equency which can excite a particular modal vibration is employed. A t hree-phase fault studies programme is used to proportion converter har monic current to each machine. This system scaling factor is evaluated for different scenarios of system operation and load. The effect of u sing different rotor equivalent circuits and the generator subtransien t reactance to evaluate system scaling factors is also shown. It is co ncluded that variable-frequency ripple currents superimposed on the DC current in asynchronous links can excite sympathetic torsional vibrat ions in turbine-generator-exciter shafts. Subtransient generator react ances may be used to approximately proportion injected harmonic curren t to each machine of a multimachine network. Very small noncharacteris tic currents could result in serious damage to the machine although po ssibility of two power systems operating with fixed deviation in syste m frequencies long enough for the vibrations to build up is remote. St udies should be converter stations to ascertain whether or not the mac hines are at risk. Monitoring or relaying systems should be employed t o protect the machine if studies indicate a machine is at risk. Machin es in Northern Ireland in close proximity of proposed HVDC converter s tations are considered in the analyses that are made.