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Dynamic Modeling and Nonlinear control of Coal-Fired Subcritical Power Generation Unit

Author: ZhuoXuSheng
Tutor: ZhouHuaiChun
School: Huazhong University of Science and Technology
Course: Thermal Power Engineering
Keywords: Bilinear fit method Superheated steam thermodynamics Radiation energy signal Coal-fired power unit model Differential geometric approach Nonlinear model predictive control
CLC: TM611
Type: PhD thesis
Year: 2008
Downloads: 241
Quote: 4
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Abstract


To match frequent varying electricity demands from the power network, the grid-connected generate units in the coal-fired power plants have to be operated over wide varying operation range. During the course, the severe nonlinearity of boiler-turbine unit, the coupling variables and time-varying parameter et al usually degrade the performance of the existing control system, sometime can confuse it and the manual regulation is switched, then the unit operation safety and economy can be badly decreased. One of the suitable methods to overcome these challenges is to develop the high-performance control system. Investigating the dynamic of the unit and modeling it are the key basal work for developing the high-performance control system.The super-heater is a significant part of the large-scale coal-firing power generating unit, and the steam pressure and temperature out of it are the major objective parameters. In the study, firstly, a new bilinear fit method which can determine the expression of the two variables function is developed and proved, then the thermodynamic property of superheated steam is viewed and the simple expressions of the enthalpy and density of superheated steam are deduced through bilinear fit. In the work, the model of super-heater is set up based on first-principle equations (mass, energy balance et al). Influence of steam mass flow rate, heat flux rate and desuperheating spray water mass flow rate on the dynamic performance of superheated steam pressure and temperature is shown in the model.The delay time constant and dynamic coefficient of the fuel system and the thermal inertia of the evaporator in boiler are the major elements which affect the boiler to response to the load command. In the study, the dynamic model of the fuel system and evaporator are set up based on the mass and energy balance equations. The furnace radiation energy signal, displaying the heat energy from fuel combustion immediately, is applied to determine the delay time constant in the fuel system and the heat flux rate to the high temperature segment super-heater. And the dynamic coefficient in the fuel system is determined by the identification technique. By analyzing and fitting, the complicated expression of the thermal inertia of the evaporator is substituted by a simple one. The expression of steam mass flow rate out of drum in boiler in de Mello’s simple model is revised and validated by using the operating data from a 300MW coal-fired generate electricity unit. Finally, the model can be validated through only using the data from a 300MW coal-fired generation unit operating over a varying operation range, and the trial is not needed. Hence, large disturbance on the normal operation of the generation electricity unit is avoided.It is very helpful to improve the performance of unit’s control system that the suitable nonlinear control methods are introduced into the control design of boiler-turbine unit. The differential geometric control and nonlinear model predictive control are of interest two methods which can be applied to the control design for nonlinear process operating over wide operation range. In the study, the differential geometric approach is applied to design the control system of a 300MW coal-fired generation electricity unit. To meet the saturation constraints on actuator, a reference governor is designed in the control scheme to make the reference trajectories sluggish. Since the unit’s model is decoupled and linearized exactly over a wide operation range, the control design can achieve tracking performance in the power output and the control system is stable.Based on Talyor series expansion to certain order, the nonlinear model predictive control approach can deduce an analytic controller and the online optimization computation burden on a receding horizon is avoided. Therefore, the control method has the obvious strongpoint in complex nonlinear process control. In the study, the control method is introduced into the control design for an oil-fired boiler-turbine unit. To solve the issue of an unmeasurable state variable in the unit, the controller is implemented in the model state feedback form, a sort of internal model control structure. In this way, on the one hand, the complex design of a nonlinear observer isn’t avoided; on the other hand, the control system’s capability to reject disturbance is strengthened. The proof shows the designed control system is robust.

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