Modeling And Disturbance Rejection Control For Boiler-turbine Coordinated Control System Of An Ultra-supercritical Coal-fired Unit

Renewable energy represented by wind and solar energy has been developed dramatically,and ultra-supercritical(USC)units need to regulate unit load under wide-load operation to help power grid absorb more renewable power and maintain grid frequency stable.Coordinated control system(CCS)is required to have strong load-tracking and disturbance rejection ability,which can ensure safe and stable operation of USC units under wide-load operation.There are characteristics for boiler-turbine coordinated system of USC units,such as nonlinearity and strong coupling,wide-load operation and many disturbances,which reduces control performance of CCS,causes superheater overtemperature and threatens safe and economic operation of USC units.To meet flexible peak shaving operation and improve operational economy and safety of USC units,this study performs research on modeling and disturbance rejection for boiler-turbine coordinated system of USC units,including nonlinear modeling for CCS,modeling for CCS with SST,state estimation,disturbance rejection method and control strategy for superheated steam temperature(SST).Specific work and innovations are presented as below:(1)Develop a dynamic nonlinear model for CCS of USC units.An identification method for nonlinear static parameters is presented to reflect the strong nonlinearity of the CCS and then improve modeling accuracy,and develop functions of mismatch between coal flow and feedwater flow and throttle loss to reflect the influence of the mismatch and variation of throttle valve opening on heat energy absorbed by working fluid and unit load,respectively.Simulation results reflect that the model can describe the wide-load operation for CCS of USC units,and it can be used for simulation analysis and control system design.(2)Present a disturbance rejection control and state estimation method based on high order sliding mode observer(HOSMO)to design the CCS with disturbance rejection.Design setpoints of CCS,according to dynamic characteristics of CCS and safe operational requirements of separator.Use the mechanism model to design feedforward and feedback actions,in order to improve tracking ability of CCS.Adopt the HOSMO to estimate the disturbances in USC units and compensate them in control law to enhance its disturbance rejection ability.Besides,coal flow in furnace is estimated to treat as the setpoint of pulverized-coal control system by combining CCS model and HOSMO,to reject time-varying and coal mass disturbances.Simulation results illustrate that the CCS can reject many disturbances effectively,track the setpoint rapidly and improve operational safety and economy of USC units under wide-load operation of USC units.(3)Develop a dynamic model of CCS for USC units with SST.Present dynamic correction coefficients of heat energy absorbed by steam to perform dynamic correction on heat energy absorbed by steam to improve modeling accuracy.Simulation results indicate that the model can describe the operational process for CCS of USC units and reflect dynamic change of SST under rapid varying load operation of USC units,and it can be used for simulation analysis and control system design.(4)Design disturbance rejection CCS of USC units with SST.Develop a novel SST control strategy that uses dynamic characteristics between feedwater flow and SST to improve SST control performance under varying load operation.On this basis,the model of CCS with SST is combined with the HOSMO to design the disturbance rejection CCS of USC units with SST.Simulation results reflect that the CCS can reject many disturbances effectively,track setpoints rapidly,maintain SST stable under varying load operation to improve operational safety and economy of USC units.