Unbalanced Operation of HVDC Converters in Asynchronous Linksin Torsional Stressing of Turbine-Generator Shafts

Abstract

This article analyzes the impact of unbalanced and balancedoperation of HVDC link rectifier bridges on ripple current superimposed on the direct current in asynchronous links in the excitation of torsional vibrations in steam turbine-generator-exciter shafts. The effect of nonideal conditions such as unbalanced rectifier ac supply voltage, unbalanced phase inductance and transformer tappings, and firing errors on the spectrum of dc harmonic voltages compared with the ideal case is discussed. These voltages give rise to dc harmonic currents at the rectifier that may excite sympathetic shaft torsional vibrations in the machine shaft.First, frequencies at which sympathetic shaft torsional vibrations would be excited by modulation product harmonic currents due tounbalanced and balanced operation of 50 Hz/50 Hz asynchronous links as a function of deviation in the system frequency are illustrated. Amplitude of shaft torsional torque due to resonant torque excitation due to harmonic current injected into the ac system by the inverter is then discussed. The article thenconsiders how the steady torque excitation depends on the noncharacteristic harmonic current that is injected into the ac system by the inverter. Tables and graphs are presented to illustrate the significance of bridge unbalances on rectifier harmonic ripple voltage (and hence on the associated currents thatare superimposed on the HVDC line current at the rectifier and subsequently at the inverter), which result in torsional torque in turbine-generator shafts. The relative amplitude of noncharacteristic currents injected into the ac system by the inverter operating unbalanced and balanced is indicated. Both 6- and 12-pulse bridges are analyzed.