Impact of shaft torsionals in steam turbine control

Shaft torsional vibrations are discussed along with their effect on steam turbine-generator control following severe supply-network disturbances and the destabilization of shaft torsional modes through the action of high-speed electrohydraulic controllers. It is shown that shaft flexibility can exert a significant influence on steam turbine-generator response following a severe supply-system disturbance, particularly when the turbine has nonlinear valve stroking and fast valving. The effect can be minimized by the careful location of a speed sensor along the turbine shaft and by filtering speed or acceleration signals to reduce the speed input of troublesome low-frequency torsional vibrations to an insignificant level. The effect of shaft torsionals on control system response to an islanding situation and on the above mentioned destabilization is evaluated. Digital implementation of steam-turbine control systems in relation to quantization, sampling, response time in protection systems, and software integrity is examined