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Combined Cycle, Steam Temperature Optimiser (STO)

Steam temperature control is one of the most critical areas of operating combined cycle power plant due to changes in power demand from the grid. The ADEX strategy aimed at controlling superheated steam temperature (T2) entering the steam turbine by adjusting the injection water flow rate. Problems of dealing with effects of temperature and flow rate changes of the combustion gases from the gas turbine, significant delays between control actions and achieving desired temperature T2, avoiding T1 reaching saturation and limitation in the instrumentation made this a challenging problem for advanced control.

Schematic of Superheated Steam Temperature Control Problems

Solution was a cascade control strategy with three controllers. The T2 controller output the set point of T1, the T1 controller output the set point of the injection water flow rate and the water flow rate was controlled using a valve. Results below show original PID control scheme compared to ADEX illustrating the effects of step increases in power demand.

ADEX 'STO' Graphic Screen Showing ADEX Cascade Control Strategy

Benefits were that set point of T2 could be increased to save energy, T1 saturation point avoided, and greater process stability/more precise control in the face of fluctuating demand.

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