Analysis And Optimization Of Coordinated Control System In The Once-through Boiler-turbine Unit

As the capacity of electric network expands and the new energy power generation may cause peak fluctuations to power grid, peak-load regulation ability of thermal power plant and rapid response to AGC demand are strictly assessed by power grid. Considering the current situation that the large units are frequently involved in peak load and frequency regulation and coal quality frequently changes, optimal design unit coordinated control system is the key to quickly respond to the load requirement and maintain energy supplying-demanding balance. The structural features of the once-through boiler-turbine unit are nonlinear, strong coupling and multivariable, which affect the disturbance rejection property and load adaptability of its coordinated system. Therefore, it is necessary to analyze and optimize the once-through boiler-turbine coordinated control system, which will have realistic guiding significance to improve the plant operation.In order to propose the control scheme of optimal operation of the unit under the condition of variable load operation, this paper firstly simplifies the overall modeling structure of the unit. Mechanism models of milling system, boiler steam-water system and turbine system are developed respectively. Through the model parameters determined with unit operation data and reasonable simplification, a non-linear model of the once-through boiler-turbine unit which has certain accuracy and is fit for the controller design for coordinated control system is built. Then the method of exact linearization via state feedback is applied to calculate the inverse system of the non-linear model. Inverse system in combination with the original system constitute a pseudo-linear system so that the linearization of the controlled object can be achieved. Coordinated control system designed by IMC structure based on inverse system effectively eliminates the nonlinear and strong coupling of controlled object. Simulation tests of the state, set-point perturbation in different static operating conditions show that the system possesses good stability and dynamic performance in a large range of operating conditions. In order to improve control system set-point tracking accuracy, a control method of feedforward-feedback based on stable inversion is proposed. The inversion solution of the non-linear model is solved by desired output path, according to the theory of stable inversion. As feedforward control input, inversion system leads the outputs to the set points rapidly and adapts to nonlinear controlled object. While the feedback controller is designed for each output variable to compensate the tracking errors caused by external interference. Therefore, the steady-state error of system output can be eliminated.