Stressing of turbine-generator-exciter shafts by variable-frequency currents superimposed on DC currents in asynchronous HVDC links and following disturbances at converter stations

Ripple currents on the DC side of both HVDC synchronous and asynchronous Links together with cleared HVDC and AC system disturbances can excite in some circumstances onerous torsional vibrations in large steam generator shafts. The problem has assumed importance in recent months on account of the HVDC Link between Scotland and Northern Ireland going ahead, on account of the proposed Eire/Wales Link, and because AC/DC/AC couplers are to be installed extensively to interconnect the East and West European Grid Systems. This paper discusses and analyses excitation of shaft torsional vibrations in steam turbine-generator-exciter shafts in close proximity to HVDC converter stations by (i) variable-frequency ripple currents superimposed on the DC currents in asynchronous Links, and (ii) disturbances at bi-polar converter stations. In (i) shaft torques in multi-machine networks are evaluated by proportioning HVDC Link disturbance currents to each machine at risk using system network data, generator data, fault analysis data, and load flow data, considering frequency dependence of the system parameters. This scaling factor is calculated for different scenarios of system operation and load. Equivalent circuits for the synchronous generator are employed appropriately to correlate HVDC Link disturbance current impressed on the generator stator with steady-state torque excitation from which magnitude for turbine-generator-exciter shaft torque is deduced. In (ii) shaft torques are evaluated by time domain computation using converter station phase currents and a faithful single machine-infinite busbar equivalent of the multi-machine power system where converter station currents are proportioned to the particular machine which is under investigation. Generator air-gap torque is evaluated by solution of the converter station/synchronous machine/infinite system differential equations. Turbine-generator-exciter torque is computed from transient generator airgap torque either in the time domain using differential equations for the turbine-generator-exciter shaft or in the frequency domain using stiffnesses, inertias and damping of modal vibrations of the shaft. Detailed (2d,3q) and reduced (1d,2q) and (1d,1q) phase-variable models for the synchronous-generator are employed. The time responses and tables show that for the systems studied variable-frequency ripple currents superimposed on the DC current in asynchronous Links can excite shaft torsional vibrations, that very small noncharacteristic currents could result in onerous shaft torques which might damage the machine, and that DC line faults at converter stations in close proximity of steam turbine-generator units can excite onerous turbine-generator shaft torsional response. Detailed simulation of the HVDC converter and generator is necessary for precise assessments of shaft torsional response following HVDC converter station faults. 500MW, 660MW, 1000MW and 1300MW machines are considered in the analyses that are made.