Optimized Steam Turbine Operation by Controlled Clutch Angle Engagement

Some single shaft CCPPs are known to display increased vibrations if engaged at certain angles and a very smooth operational behavior if engaged at other angles. Neither the reason nor a mechanical mitigation for this behavior is known.Roughly, the engagement angles can be clustered into four 90° sectors, one of them is deemed unfavorable and another one is classified as favorable with respect to vibrations of the overall power train shaft. Although the first priority is to have an assembly that runs smoothly at arbitrary engagement angles, an option to engage at a predefined ‘good’ angle provides an I&C back-up solution to avoid undue vibrations.To meet this requirement a control method was developed to run up the steam-turbine thus that it will engage at a pre specified angle. The basic idea of the developed control algorithm is to increase the acceleration of the steam-turbine during run up if a prediction reveals an engagement angle that would be too small to be within the desired area and, conversely, decrease the acceleration if the anticipated angle would be too large.A control algorithm to exploit this idea was developed and tested with the aid of a simulation model. The simulation model comprises a detailed thermodynamic model of the steam-turbine, the control valves and their actuators, the steam-turbine shaft and the shift clutch. I&C was modelled with all details relevant to turbine speed control. The gas-turbine/generator unit, however, is cut down to angular velocity and relative angle, respectively.The performance of the controller was validated to comply with parameter uncertainties, different sampling times and frequency fluctuations of the electrical grid during coupling. In parallel, a high precision angular measurement device was developed and integrated into the existing turbine I&C.Hardware-in-the-loop tests with simulated turbines and a hardware I&C system implementation also revealed a very satisfactory performance.On site implementation of a prototype was successfully accomplished and resulted in the predicted accuracy of the preset engagement angle.Copyright © 2015 by Siemens Energy, Inc.