The Study On The Control Of Coal-water Ratio And Main Steam Pressure In Supercritical Units

The main steam temperature and pressure are the key control parameters ofsupercritical unit coordinated control system. The stability of the boiler steam parametersdepends on the balance of the amount between coal and water, and the balance betweenthe steam turbine and boiler output power. So the stability of the boiler steam parametersessentially depends on the quality of the coal control.Supercritical once-through boiler can be regarded as an uncertain system, because ofthe changes of the parameters under the condition of variable load operation. A modelwith three inputs and three outputs of600MW supercritical unit was simplified to be alinear approximation model with two inputs and two outputs, with the method of partiallydecentralized control. So the decoupling control of supercritical once-through unit can berealized. And the rationality of separate modeling based on the effects of dynamiccharacteristics of fuel quantity on the intermediate point temperature was demonstrated.By taking the effect of fuel change on outlet temperature of water wall as the object ofthe study, the dynamic mechanism was analyzed and the transfer functions under threetypical working conditions of50%THA,75%THA,100%THA was established with thelumped parameter method. Then, the variation range of the model with uncertainties wasdetermined. The feedback controller and pre-filter in the fuel-water ratio control weredesigned, by using quantitative feedback theory and combining with the given designperformance. Simulation results show that boiler intermediate point temperature cantrack the given signals rapidly with highly precision. The designed controller which hasthe strong robustness performs well.H∞optimal control theory which has strong ability of interference suppression androbust stability can effectively solve the problem existing in the steam pressure control ofthe supercritical once-through boiler. By taking the H∞controller design on the controlsystem between fuel and steam pressure as a mixed sensitivity problem, the weightingfunction was chosen according to the design performance index, and the H∞controllerwas obtained by applying H∞control theory. The singular value analysis and thesimulation results show that the designed H∞controller which has strong robustness caninhibit the external interference signal of the load effectively and improve the tracking precision of the control system fully.